It doesn't happen often, but some days Dean wakes up first. He uses the time wisely. Opens his eyes to see the broad expanse of Sam's back, and he turns her over gently enough that she doesn't wake so that he can take inventory.
There are a million changes in her every day, some of them big, some of them minute, the kind of thing even the naked eye can't see. Dean can, though, because it's Sam. He has every atom of her memorized.
She changes so quickly these days. It used to scare him. Used to make him panic, because if she keeps shifting, how can he pin down any version of her? She was a caterpillar once, crawled off scrawny and self-conscious. She didn't stay that way for long.
He used to resent it, because he wasn't there to see it begin. He walked into a dive bar in Palo Alto looking for his brother, was told Sam Winchester worked there, and got distracted by a tall drink of water in a pink tank top and a tight jean skirt instead. When he sauntered over with his cocky grin and asked to buy her a drink, it was his baby brother's face that looked down at him through mascara and eye shadow, and Dean didn't know what he was looking at.
That girl was not the boy who left him for bigger and better three years earlier. That girl is not the woman who's lying in his bed right now.
Every day it's a different Sam. He didn't understand at first, but he does now.
He was angry that night at the bar, not because this is who she is, but because she hadn't told him. That Sam could have hidden something so big from him, could have cared about him so little—it was Dean's worst nightmare come to life. But he knows now, even though she's never told him. It was never about him. It was about her. How scared she must have been, and Dean never seeing it, never knowing she was trapped inside tattered hand-me-down hoodies and too-small boys' underwear. He didn't see so he didn't free her, and his instinct was to hate her for freeing herself. He's not proud of it. Not proud of a lot of things Sam makes him feel, but he's proud of Sam.
She escaped because she had to. Because Dad would never have even given her a chance to fully understand who she is, let alone to become a sweet, compact version of the warrior their father raised. Still deadly, sharp, smarter than any hunter her age and most out there twice her age, too. But all that with shoulder length hair that gets in her eyes and manicured nails she'd rather not chip. The extra half hour for make-up, the out-of-the-way stops they make every month so that Dean can steal her a bottle of hormones—those things would never have flown with John Winchester. Frivolous, he would have said. Getting in the way of the job. "Is your lipstick more important than saving lives?" he would have demanded, and Sam would have said 'no, sir,' and it would have cost her her life.
Sam never could have become Sam if she hadn't gotten away. As much as it hurt Dean to lose his brother at the time. As much as it hurts to lose her every night, only to fall even deeper in love with the woman she is in the morning.
He charts the changes every time he gets a chance like this, her wider hips, the softness of her skin, curves where there used to be lean muscle. He keeps a list in his mind because a part of him still worries, even now, that he won't be here to see how it ends. Girls like Sam, they don't stay with guys like Dean for long. They’re meant for so much more.
But for now, she wakes slowly, blinking her slanted hazel eyes at him, and she smiles. She lets him kiss her lazy and pliant until she's as desperate as he is, grinding up against him, wrapped in his arms.
"Dean," she says, and, "Need you," just like she did as a kid, but she's asking for something different now.
He cups her barely-there breasts in his hands, gentle, because they're still growing and they’re sore, but they're getting so sensitive these days. When he pushes her shirt down so he can take one between his lips, she gasps like she's got a live wire running straight down to her cock.
When he reaches into her cotton panties and begins to stroke her, mouth still latched to her tit like he's nursing from her, Sam curses and arches up into him. Nothing in Dean's life, not the thrill of a hunt well done, not even the purr of his baby on a long stretch of road, will ever measure up to the quiet mornings like this, when he wakes his little sister with hands and mouth until she's shaking, coming apart in his grasp.
She rolls over when they finish, wrapping the comforter around herself. "Five more minutes," she mumbles from inside her cocoon, just like she did when she was a bony thirteen year old boy with acne and racecar pajamas.
Dean watches her after, too, when she drifts off in her afterglow, and it's nearly painful sometimes to look at her, to see something so beautiful all over and know that she's his, but that she might not always be.
He has a list a mile long of the differences: hair length, tone of voice, which movies make her roll her eyes, where he has to touch her inside to make her cream for him. But there's one change that matters more than the rest. When he watches her like this she smiles, and it's easy, unguarded. She doesn't scream as she jolts awake with nightmares anymore. The dimples come more often, her eyes are brighter.
He's learned to let her go, because that's the only way to be sure she comes back. He's gotten addicted to watching his little sister flap her butterfly wings and fly.