The thing about having a little sister who’s also a pet demon is that there’s a lot of things she can’t do. Dress herself. Clean herself. That kind of thing.
Or perhaps just won’t do. Tanjiro could never decide on which he thought was more likely. On one hand she could take out entire demons, flinging herself at her enemies and contorting her body in ways Tanjiro had never seen before. And then on the other hand, she struggled to lift her arms high enough for him to pull her kimono off. Maybe it was from the exhaustion, maybe simply from embarrassment. Tanjiro couldn’t place it.
Besides in those moments, when Tanjiro helped her undress, he tended to be, let’s say, a little distracted.
He was still only a young adult afterall, he had… needs. These needs were usually met a few minutes after he had finished helping Nezuko change. He’d always make sure she was asleep before he did anything though, he could tell by the snores. The soft and gentle snores of our Nezuko.
They’d sleep together most nights. No matter where they happened to be staying. If she fell asleep in the box, then that’s where she slept. If she was still awake however, Tanjiro would always offer her his hand to guide her to his side. And on this particular night that’s where she slept, dozing off with her arms held tightly round his side. Pressing her bare chest into his left arm. Seemed as if her nipples were constantly hard. A side effect of her demon nature perhaps? Or maybe just the cold? Either way, he wasn’t complaining.
He and Nezuko slept in the nude most nights, with only Nezuko’s haori draped over them. It was something they’d done since they were little kids. It was comforting. The bare warmth of another’s skin against his own. Meant they weren’t alone. And even now, with Nezuko’s skin icy cold to the touch, it still felt warm to Tanjiro.
Yet of course people, as they often do, grow up. People develop sexual urges and desires. What was once just a comforting gesture becomes something more. Something balancing on the thin line between the norm and the taboo.
Tanjiro had grown to accept this. Learned to dangle over that line when he was on his own. He was ashamed to even think it and he’d never admit it to anyone but he had thought about Nezuko in this way since before the tragedy. How could he not? She was simply a beauty and he was but a young boy. One who knew many girls but never felt that intimacy and desire he felt with Nezuko with any of them.
He had gotten a few glances of her body, of course. It was hard not to see your sister’s private parts at some point to be honest. Anytime a girl sat with her legs apart you’d catch a glance. They wore nothing over it. That’s how he justified it at first.
He had only started touching himself to the thought of her since the day Sakonji brought him in. There were nights where that’s all he did. He’d always regret it in the morning, but damn if there wasn’t a small voice in his head telling him it was all worth it.
Then, when he became a Demon Slayer, it was turned up a notch. That’s when he started touching himself beside her, as she slept.
Countless times he’d done it now. Never quite sure if she was aware or not. Yes, he was careful but he never knew how careful was enough around a demon.
“Nezuko...?” Tanjiro whispered into the night air.
No response. No grunt. No soft moan. Nothing.
He just had to be sure.
The feeling of her body next to his was enough to get him hard. He was probably hard since he undressed her, he wasn’t sure. His lower body ached for him to move, for him to thrust in her direction. But he kept like hips still.
Slowly, he guided his hand down stomach to his cock. Turning his head slightly to gaze at Nezuko, he started to stroke very gently. His fingers just about touching his cock. Each time grazing the head and making him want to give in and just pump it out.
But he refrained.
He could see his breath hitting against Nezuko’s cheek in the cold air. It almost drove him insane to see part of him getting that close to touching her. Could she feel it, the hot air, getting ever hotter by the second, spreading across her face? She could be aware of a lot of things in her sleep, it wouldn’t be a huge surprise if this was one of them.
What would she think, knowing her brother was touching himself because of her? That the heavy breathing which occasionally hitched and flowed into moans was because of her?
Tanjiro’s mind ran wild with scenarios. Every last of them completely unrealistic but absolutely heavenly.
His eyes shut as he tugged on his cock at a quicker pace. Images of Nezuko’s tits being groped with rough hands flooded his mind. His body arched forward and his legs folded in slightly involuntarily. Left leg touching Nezuko’s but Tanjiro hardly noticed. He couldn’t. He was far too involved at the moment.
“- Nezuko…” The words escaped his mouth and for a reason he would never be able to explain, that was okay. He was allowed to be loud. To be gross. He allowed himself. He wasn’t sure why. “Nezuko, please -”
His eyes still shut, his left hand grabbed a hold of something. Why was his hand moving? He didn’t know, it just was. He couldn’t hold himself back any longer. He couldn’t.
His entire body flipped and his eyes opened. He was right above Nezuko. His left hand on her boob. His right hand still stroking his cock, harder than ever. The realisation of what he was doing finally dawned on him. He was doing the unspeakable. He was right in the middle of the act. He couldn’t back out now. His hands swiped away from Nezuko’s body and his own for a moment.
But then he caught her eyes flickering. A soft yawn could be heard as she rubbed them, struggling to awaken from her abruptly cut off nap.
Her eyes focused and suddenly the fear and panic that had filled Tanjiro was gone.
“Nezuko! I- ah-” He was just about to explain, propose some lie about what this was or maybe simply encourage it to continue. But he couldn’t. The words didn't get out in time.
Just as his mouth was about to open he felt something pressing against his asshole.
His eyes flickered down and he saw two things. First, his cock, still hard as ever, pre-cum coating it. And second, what seemed like a cock protruding out of Nezuko’s pussy, but to be honest Tanjiro wasn’t sure what he was looking at. It was shaped like a cock but how exactly could it be--
He never saw the head. It was inside him before he could.
Tanjiro felt his asshole stretch in a way he’d never felt before. He wanted to scream, to cry out. He wanted to get some help but who would help him now? How would they? Even if they could, that might put Nezuko in danger.
He didn’t want that.
Holding back tears, he bit his lip and endured.
Nezuko eyes seemed full with lust and if this was any other scenario that would turn him on but- Hell, he was turned on! He couldn’t deny it. His cock was hard and his moans of pleasure probably alerted any demon of their presence in 5 mile radius. God, fuck it. He loved this.
His arms stretched out as his body gave way and they wrapped around Nezuko’s neck. Then she started moving in and out. Each thrust drove him to the brink of cumming already. Her intense breathing right next to his ear almost did the same.
Drool fell from his lips onto to her shoulder as his body vibrated with each pump into him.
“Ah Nezuko, slower, please-”
She kept going, ignoring his requests. Tanjiro wasn’t even sure she heard him. He was pretty certain all she could think about right now was fucking the shit out of him.
Tanjiro’s hips started moving in sync with Nezuko. His breath hitched countless times. He could feel it building up and he was going to ride it out screaming.
“Nezuko. Nezuko, Nezuko!”
He fell flat onto a puddle of his own cum on her stomach. Arms still tight around her neck and fingernails digging into whatever they could.
And then he suddenly felt empty. Nezuko was no longer inside him. It was over and he couldn't help but feel like there should be more. But god, for the first time in ages, he felt so complete.
“Nezuko… that was amazing!”
And… she was asleep.
Well, suppose fucking the brains out of your brother is likely a tiring activity.