When Dean is very tired, and even the sound of Sam breathing deep and soft in the bed next to him (and thank God he doesn't snore, because Dean would have smothered him with a pillow years ago if he had) isn't enough to get him to sleep, then he thinks strange things. As with most things in his life, half the time it's about Sam, the remaining half evenly divided between hunting and sex.
Tonight, it's Sam. Well, not Sam specifically – some weird alternate Sam, who happens to be a girl. Dean is trying to imagine what having a sister would be like. So far, he's settled on that it would suck, a lot. He can't quite lay his finger on why, but it would suck.
It might be nice if Sam-the-girl were short and petite, Sam is, for someone who's sixty-three feet tall, pretty stealthy, but still, it would've been good to know someone who could get into those tight spots where people like to stuff bodies, and creatures like to hide. But then, a short, petite girl would be even more crap at avoiding kidnapping than tall, brawny Sammy, and Dean would spend even more of his time worrying then.
It occurs to him that Sam-the-girl wouldn't share a room with him, their family is fucked but Sam's prudish enough when he's a guy – imagine if he had any interesting bits to keep hidden (well, not interesting to Dean, but Sam's like that already, probably 'cause he thinks Dean'd make fun of him). Dean shudders. It's four times a night – four times a night Sam'll never know about – that he's up to check that Sammy's still breathing (because he's Dean and that's actually something that worries him) and hasn't slipped away in the night ('cause, fuck, that's happened before).
Then, something drifts across his mind – a half-sleepy thought, one of those things that you can't control, like when you know the stove's off, but all you can see is the gas switched on and nothing else, so you go downstairs to check four times because it just won't stop playing in your mind- if Sam were a girl, he'd be easier to control. If Sam were a girl, Dean could actually kick his ass, not just because Sam's still a little out of practice and because Dean's still his big brother and even if Sam has got fifteen pounds and four inches on him, he'll never use it because it's hasn't honestly occurred to him that if he did he'd win every single time. If Sam were a girl, then Dean wouldn't have to think about firing the guns into Sammy's skull every time he cleaned them, the look on his brother's (if it comes to that he won't be your brother, he reminds himself, because he knows that if there's even a little bit of his boy left, then he won't be able to do a damn thing but try to get him back) face when he pulls the trigger, the weight of the gun in his hand, the sound of Sam's last breath.
It's like it's already happened. But it hasn't and it won't.
He goes back to the much more peaceful girl-Sammy thoughts, leaving that one to lie at the back of his mind, where he knows it'll stay for another few days, and then come back to haunt him when they're driving in the ass crack of Texas and Sam is asleep – impossibly small for such a large person – in the seat next to him, and Dean gets that old impulse to run his hands through Sam's hair, or Sam snuffles in that little way he has or does something else that makes him five again in Dean's eyes (not that Sam's ever really gotten much older than five for his brother). That's always when his father hisses in his ear the most strongly.
It occurs to him that he's never actually been able to say no to Cassie or Sam, and he's no good at arguing with women most of the time. He figures that Sam might not have had to perfect the puppy dog eyes to get what he wanted if he had been a girl. He-she would have just stood there and asked and Dean would have caved (it does not occur to him that that's normally what happens anyway).
Sam rolls over, one long arm flying out so it almost reaches Dean's bed, doing that snuffling thing, and being altogether too large for his double bed. He turns his face towards Dean, and Dean remembers why it would actually suck to have a sister – 'cause even if, really, most of the half of his time he spends thinking about Sam is spent worrying about Sam, and if he can hear bullets going off in his head, and he hates Sam sometimes anyway, he's his brother and Dean knows that he would be at least twice as fucked up if he didn't have Sammy (who's done a good deal of the fucking-up himself). He figures that even Sammy in another form wouldn't quite do it, he'd never really be the person he actually is, and he wouldn't love Sam quite the same way that he does. He doesn't care that (and here is a thought he does not allow himself, even in this mood) if Sam were anyone else on earth, maybe (probably) Mom would still be around – but he wouldn't have the one thing in his life that he knows. Beyond a shadow of a doubt knows that he has to take care of Sam no matter what, love Sam no matter what, protect Sam no matter what.
If there wasn't Sam, just as he is, too big and too small at once, stronger than anybody (or more powerful at least) but still asking stupid questions and looking up to Dean on the rare occasion, there wouldn't be Dean. At all.