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You’re relieved to see your family gym’s not very busy tonight. In fact, you’re the only patron there, if you really even count. Coming in after school to work out, for free, doesn’t exactly mean you’re a customer. The only other person in the building is your older sister, Ayaka, who is currently running the counter. Though, right now, that more or less just means trying to keep herself occupied. 

Your father started up the gym as a place to train and prepare professional boxers, more than just general workouts, and it’s something your whole family gets pretty into. Ayaka is a competitive girl, making it much further in her training than she probably would have otherwise on sheer competitive spirit. Nana, your eldest sister, is a target Ayaka is always striving to reach and surpass. You didn’t fall into that side of things as much as the rest of your family, more eager to watch how hard your sisters train, especially Ayaka, but recently you’ve been heading to the gym more often.

You have similar hobbies to Ayaka’s other hobbies, gaming and action movies, and while you’re still young, the lack of real activity in your day to day life has been taking its toll. You’d like to get in better shape, and working out at the gym is a great way to do that. Especially when it’s just you and Ayaka here.

“Oh, come on, you can handle a couple more sets, don’t you think?” Ayaka calls out from the counter, as you stop to take a breather. She grins, and you know she’s egging you on because she’s bored. “Don’t make me turn this into a contest!”

“There’s no way I could keep up anyway,” you answer, chuckling. 

“Well, not yet, but if you keep at it maybe you can!” Ayaka responds. “Don’t sell yourself short, little bro, you’re still part of the family! If dad hadn’t mellowed out after me and Nana, you’d really be in for it.”

“You’re probably right,” you reply, taking a drink while you catch your breath. “Guess I lucked out, huh?”

“Yeah, I guess so,” Ayaka says, stepping out from behind the counter. “But I can always push you harder, if he won’t!”

Due to the nature of the gym, it’s unlikely they’ll have any customers coming in so close to closing. Most of their members are here for on hands lessons and training, and it’s rare that they come in just to work out. You’re the exception, to be sure, but that means Ayaka can afford to pay more attention to you. You wonder if Hibiki is busy, usually she’d be glued to her phone discussing whatever movie they last watched.

“Oh, that’s okay, you don’t need to worry about that!” you say, but secretly, you’re glad Ayaka is wanting to play personal trainer.

Slow nights like this are your favorite, especially when Ayaka decides to spend some time with you one-on-one. The two of you have grown closer over the years, often watching movies or gaming together if Ayaka’s friends are busy. She may have been settling, but you’d never complain about a chance for some quality time with your big sister. Her training has always kept her in fantastic shape, and you’re far from blind. Perhaps you shouldn’t look to your sister like that , but you’ve never found that easy. Especially as you grew closer, and closer. 

“You say that, but I know my brother,” Ayaka says, winking, “you’ll be out of here in no time if I’m not pushing you. So, come on! Do another set, for me!”

She’s right, of course. You might be trying to get in better shape, but you’re still nowhere near as determined as you might be otherwise. Having Ayaka pushing you, challenging you, does always help you get a lot more done. You get back to it, eager to try your hardest with Ayaka watching. You probably would have ducked out a bit before closing, had Ayaka stuck to fiddling on her phone, but like this you’ll definitely make it the last hour. 

Each time you stop to rest, or decide to move on to something else, Ayaka is right there goading you on. There is still some equipment you’re not familiar with, and some exercises you know Ayaka is great at that you’re hopeless with, so you will occasionally ask her to show you how to do them. It’s a great excuse to catch your breath, with an added bonus of getting to watch Ayaka work out. 

You have to be careful not to lose yourself in that too much, this may be your family’s gym, but without it being locked up, there’s no telling who might walk in. Getting caught checking out your sister when she’s supposed to be demonstrating an exercise wouldn’t exactly look good, and if it was your dad or Nana catching you, you’d be screwed.

That doesn’t mean you can’t enjoy the view at all, though. Of course, it’s never long before Ayaka is barking orders at you again, motivating you to get back to work. She likes showing off, for sure, but she’s not going to let you slack off, either. It’s a fine balance, and one you’re perfectly fine with. You really do want to get into better shape, after all, and this sort of reward for your efforts is a nice added bonus. Seeing Ayaka’s taut, toned body working out has never been easy on you, and the both of you have fallen to that temptation before. You wonder if it’s not intentional, how she’ll show off for you, but you’re not going to ask.

It becomes a rhythm, working your ass off while Ayaka urges you to do more, just a bit more, to push yourself that little bit harder, and then catching your breath while she shows you something new. Alternating like this, time passed quickly. Ayaka isn’t giving her all, for sure, but it’s been a long shift. She’s been keeping herself hydrated, and has been the only employee here for quite some time. She would have headed here as soon as she was out of school, while you meandered and killed some time first, hoping the crowds would die down.

She hasn’t had much of a chance to excuse herself, and you’ve kept her busy for a while now. She does her best to hide her condition, but you’re so used to watching her, you can notice the way she fidgets from time to time. Now, when she is going through her demonstrations, she’s a bit slower, not showing off as much as usual. You can tell she’s starting to get a bit desperate, that it’s only a matter of time before she excuses herself to use the bathroom. 

For some reason, you don’t want her to. You want to keep watching her like this, to see her squirm. It’s a new feeling for you, unfamiliar, but you think you know just the way to do it. For now, you’ll keep waiting. There’s still some time before closing, and you can use that to your advantage for sure. Ayaka never turns down a challenge, even if it’s one she has no chance at winning. And that’s exactly what you’re banking on tonight.

“Hey, can you watch the counter for a sec?” Ayaka asks, eventually. It would seem her need has grown too great for her to completely ignore it. You’re not exactly used to working in the gym, unlike Ayaka and Nana. But, you understand the basics and could definitely handle a couple minutes while Ayaka goes to relieve herself. You could handle it, but you decide to play your hand instead.

“Oh, come on, Ayaka,” you start, “I think you can make it.”

“Huh?” she asks, shocked. She didn’t think you could notice how bad off she is getting, the way she keeps fidgeting… but she bounces back quickly. “Well, you’ve got me figured out. So, what, you want me to try and keep holding it?”

“Yeah, it’s like some endurance training or something. A challenge,” seeing the ways her eyes light up when you say challenge, you know you’ve got her. “If you can make it all the way until closing, you win.”

“That’s easy, and winning is great and all, but what would I win?” Ayaka asks, placing a hand on her hip. “If there’s no stakes, it’s not a very fun challenge, is it?”

“That’s true,” you admit, already knowing she’d want to spice things up a bit. Ayaka is not one to refuse a challenge, sure, but she also wants them to be as much fun as possible. “Well, what if we exchange favors? If you make it-”

“‘ When ’ I make it,” Ayaka cuts in, correcting you.

“If you make it, I’ll owe you one. And if you win, then you’ll owe me a favor,” you explain, and she seems interested.

“Hm, so, what exactly are these favors good for?” Ayaka wonders, “Like, they’ve gotta be a pretty big deal for this.”

“Anything,” you say, firmly. “Anything at all.”

“Okay, fine. But only because this is going to be so easy. The gym’s only open for thirty more minutes anyway!” Ayaka says, grinning confidently. She thinks she’s already won, and there’s honestly a good chance she will. Ayaka isn’t exactly one to be underestimated, but even if she does win, you’ll gladly take that risk if it means getting to watch her squirm and fidget for a bit longer.

Sure enough, as the two of you fall back into your pattern of you exercising, and Ayaka demonstrating, you can see the strain more and more clearly. She isn’t as eager to drink from her water bottle, either. Before, she was making sure to stay well hydrated as she worked, even though she wasn’t doing anything particularly intense. Now, you can tell she’s avoiding any unnecessary risk. Of course, she’s already feeling her earlier efforts at hydration as her bladder fills more and more.

Time ticks by, and with each minute she’s that much closer to victory. But, as victory grows nearer, holding on becomes that much harder as well. It gets to a point where she can hardly stand still, and you decide to take a break. You sit down, honestly exhausted from your efforts, and reach into the bag you brought. If Ayaka is trying to avoid having a drink, things must be pretty bad for her. Naturally, you’re not afraid to use that to your advantage.

You pull out your own water bottle, drinking heavily in front of your older sister, while you watch her squirm as she struggles against her aching bladder. Neither of you set any sort of ground rules, so you don’t feel bad about working to put some extra pressure on her. You know she’s not going to call you out on it, if she did it would only show just how bad off she’s gotten. Instead, she adamantly looks off to the side. If she’d brought her phone over, rather than leaving it at the counter, she’d probably make it a point to scroll through and ignore you as much as possible.

No matter how hard she tries to ignore it, you can still tell it’s having an effect on her. Slowly, she crosses her legs. Then, pushes a hand between her legs. By now, she only has to last about 10 more minutes. It’s nothing compared to how long she’s already waited, but her body wouldn’t agree with that assessment. Her overfull bladder must ache, and she is doing her best not to show it. 

“How’s it going, Ayaka?” you ask, earning a glare.

“Shut up,” she answers, “Let me focus.”

“Wow, you’re having to focus? I thought you were definitely going to win,” you continue, keeping your tone light and teasing. She doesn’t respond, instead looking down at the floor. The gym’s floors aren’t anything special, but the way she stares you’d think it was abstract art. 

Silence falls over the two of you, as the seconds tick by. You check your phone, and see it’s already a couple minutes closer to her goal. 

Again, you don’t really see any way to lose in this situation. Even if Ayaka thinks of the most embarrassing thing possible to make you do, you’ve still been able to watch her struggle for almost an hour. It will be a photo finish, but that really only adds to the thrill. Each minute is a thrill, as you wonder how things will play out, but you know it must be agony for Ayaka. Just as you start to think she really might make it, your sister lets out a strangled squeak.

“Ah, no!” she cries out, and your eyes dart down. You can’t see anything, with her legs crossed and hand shoved between them, but it doesn’t seem like she’s lost control yet. You wonder if she’s leaked a bit, finally finding it impossible to hold on completely.

“You okay?” you ask, as she tenses up. She simply nods, eyes screwed shut as she struggles against her body. Checking the time again, you see she’s only got a couple minutes to go. Not that that seems possible, given her current state.

It’s no surprise when she cries out again, even more frantically than before. Her other hand darts between her legs, trying desperately to stop the flood that’s threatening to overwhelm her. Of course, it’s a fight she’s destined to lose. You take a lot of pleasure in the knowledge that, with how bad she’d let things get, even if she had managed to last until closing there was no way she’d make it to the bathroom. 

“Ayaka?” you ask, watching eagerly. There’s still a minute before closing, but you can till this is over.

“No, no, no!” she cries out, but her refusal makes no difference. Her strained bladder gives in, and you watch eagerly as a dark patch begins to spread through her shorts. Before long, she’s soaked through them, and her piss is streaming down her legs in rivulets, before splashing onto the floor. Her head sinks in defeat, even as she continues pissing herself. You’re honestly impressed at just how much she held, knowing she must have been in absolute agony towards the end. 

Finally, she finishes, and you approach her. You reach out, placing a hand on her shoulder. She doesn’t look up at you, sulking as she feels the sting of defeat, and the humiliation of pissing herself in her family’s gym, with her brother watching. 

“I’ll go lock up, okay?” you offer, after a moment. It seems to stir her from her funk, a bit.

“Yeah, thanks,” she answers, lifting her head. “I guess you won, huh?”

You did win, but you’re not about to rub it in. It was great, seeing Ayaka lose control like that after struggling for so long, but you don’t really want her feeling bad about it. You go about locking up, while Ayaka focuses on getting the mop together to clean up. You can’t stop thinking about it, the way she stared down, fighting to the last moment, only to wet herself right in front of you. It’s something you’ll likely think about for days, even weeks to come, surprised to find you like it even more than you thought you might. 

Once the doors are locked, and everything is straightened up, you come over to check on her again. She smiles up at you as you approach, and determination is shining in her eyes.

“I may have lost that challenge, but things won’t go your way next time,” Ayaka promises, already mostly back to her usual self.

“Next time, huh?” you ask, and her eyes widen briefly. “Didn’t think you’d want to do that again.”

“Well, I need to prove myself. Today was just a fluke, but if we do this endurance training thing again, I’ll make it for sure!”

“Alright, we’ll see next time, then,” you agree, shocked but honestly thrilled. Did Ayaka… enjoy that? Or was it just the thrill of the contest? It’s sometimes hard to tell with Ayaka, sometimes she doesn’t care for something at all, but with high enough stakes or enough competition, she’ll get really into it. 

“Either way, I’m soaked, and I’m taking a shower before we go,” Ayaka says, once she’s nearly finished mopping up. “You can head on home if you want, I’ve got it from here.”

“Well, actually,” you reply, deciding to press your luck a bit further tonight. “I’d like to go ahead and use my favor now.”

“Eh? Already!?” Ayaka asks, stunned. “On what?”

“Let me shower with you,” you say, firm enough that she knows it’s a serious request.

“H-here!?” she asks, surprised you’d be so bold. 

This is far from the first time the two of you have pushed the limits of your sibling bond, and it wouldn’t be the last, either. You find Ayaka irresistible, and she seems to be into you as well. But, Ayaka’s worries are grounded. This is your family’s gym, and your eldest sister or father could come in to check on what’s taking you and Ayaka so long. And if the two of you are showering, together, you’re not likely to hear them coming.

“Yeah, here,” you say, anyway. It’s risky, without a doubt, but you can’t stop staring at Ayaka, can’t get how badly you want her out of your mind. If you were to just leave, you’d think of her all night, the way her shorts look, completely soaked… the thought of her cleaning up in the showers… it would all be too much, so you have to take the risk.

“Well, I guess I have to do it, right?” Ayaka agrees, after a moment of thought. She smiles, just a bit, and you wonder if she’s not a bit excited at the thought, too. “After all, it’s your favor, yeah?”

“We can just text dad and tell him we got caught up cleaning,” you say, relieved she agreed so easily. “Then I doubt he’s going to come check on us.”

“Ooh, yeah, smart! Alright, I’ll let him know you’re staying to help me, and I think we’ll be fine,” Ayaka says, seeming to be fairly excited by all of this. By now, you’re sure she’s eager to get stripped down and into the showers, her shorts must be feeling cold by now. It doesn’t take her long to fire off a text, and the two of you are headed for the showers. 

You go ahead and turn the water on, wanting to let it heat up as the two of you undress. You watch intently as your older sister tugs her shorts down, along with her panties, marvelling at just how soaked she is. It’s hard not to make it obvious how excited she’s making you, considering you’re working your way out of your own clothes. Before long, both of you are naked, and stepping into the shower stall you’ve gotten warmed up. Ayaka goes in first, definitely the one in most need of a shower. 

You follow, and can’t help yourself from pressing up against her. You wrap your arms around her, placing your hands on her toned stomach, leaning in to kiss her neck. You wonder if it wouldn’t have been better to give her a chance to clean up first, before moving in on her, but honestly, you weren’t sure if you could wait that long. After watching her for so long, being this close is unbearable. You need Ayaka, and you don’t want to wait any longer than you must.

“You’re eager, huh?” she asks, as you guide your hands up, eventually cupping one of her breasts in each hand. You slip your fingers around her nipples, teasing at them, earning a moan from your sister.

“You love it,” you respond, and she doesn’t answer. You watch her bite her lip, and know you’ve got her beat. You continue toying with one of her nipples, but drop your other hand between her legs. You work a finger inside of her, finding that she’s already nice and wet, and it’s not just the shower, either. “You’re excited too, huh?”

“Maybe I am,” Ayaka says, “So don’t keep me waiting, okay?”

With that, you turn her around. With her facing you, you push her back, against the wall of the shower stall. You’re glad you’ve spent so much time in the gym lately, both because you’ve managed to get much closer to Ayaka, and because you’re strong enough to support her as she braces herself against the wall, spreading her legs and locking them around your back. You hold onto her, as you push the tip of your cock against her cunt. She looks at you, biting her lip again, and you know better than to hold back any longer.

You thrust in, slowly at first, letting your sister get used to having your cock inside of her. Ayaka could probably handle it if you were quicker, rougher, but you want her to enjoy herself as well. That’s likely a big part of why she’s enjoyed spending so much time with you lately, letting her little brother fuck her; you love seeing her happy and do your best please her. You fall into a steady rhythm, doing your best to keep pace as you thrust into her. She’s so tight, as always, it’s hard to bear, but you aren’t about to give in. 

Ayaka’s competitive nature shines through at any opportunity, and sex is no exception. She always tries to hold out for as long as possible, always wanting to outlast you, but this is one arena you refuse to lose in. At least, refuse to lose without a fight. Tonight will be a struggle, with how excited you’ve gotten watching her all evening. Before long, your steady rhythm starts to collapse, even as Ayaka begins to whimper at each thrust. You can tell her resolve is starting to weaken, that you have her close, but it’s so hard to hold on. She’s not making it easy, bucking her hips against you frantically even as you fuck her, whatever rhythm you once possessed fully shattered as the two of you lose all semblance of control. 

You do your best to focus, to take your mind off how close you are, but it’s a useless battle. With a groan, you thrust into Ayaka one last time, before you’re at your limit. You can’t hold out any longer, and you only barely manage to pull out before you come. Knowing she’s won, Ayaka’s own resolve crumbles. She cries out, as you keep supporting her, and it would seem her bladder wasn’t fully emptied earlier. She loses control again as she comes, squirting on you as the throes of her orgasm overwhelm her. Feeling the hot spray of piss as she squirts, not quite overwhelmed by the spray of the shower, is a thrill you did not expect. 

“Ayaka,” you murmur, finally letting her down. She takes to her feet, only a bit shaky after her climax. “That was amazing.”

“It was pretty fun. But I won, didn’t I?” Ayaka asks, grinning. “So, do I get a favor, too?”

“Huh?” you respond, a bit stunned. The two of you hadn’t set up anything official, but it is true that she lasted longer than you. “Yeah, you did win fair and square.”

“Oh, good. Well, I’m going to go ahead and cash mine in, too. Next time the gym is slow, you’ve gotta come over so I can ‘endurance train’ with you again, alright?”

“Really?” you ask, hardly able to believe what you’re hearing. “You’d want to do this again?”

“Yeah! Honestly, it was a great time,” Ayaka admits, blushing a bit before she turns away, focusing on getting herself cleaned up. “But next time, I’m going to make it, alright?”

“We’ll see. I’m hoping you don’t, and not just so I win again,” You reply, admitting just how much you enjoyed this. 

“Yeah, I know,” Ayaka chuckles a bit, “You liked seeing me piss myself, huh?”

“I’m that obvious?” You ask, shaking your head.

“You hardly stopped staring all night, little bro,” Ayaka answers, shrugging. “Well, if we’re both having fun, there’s no need to worry, right?”

The two of you finish up your shower, and get changed, knowing if you take too much longer someone is sure to come check on you. You’ll have to get back home soon, and pretend none of this happened, at least until the next time you have the gym to yourselves.