Dean's the one who introduces them. Well, sort of. He gets a job at a small construction company a few weeks after they settled down, making use of the only marketable skill he has aside from his knowledge of cars, and Rachel's the secretary there. She's nice; they get along well enough, joke around, have meaningless little work conversations.
One afternoon, about two months in, Sam picks him up after work. The boss calls Dean in as they're about to go, to talk about next week's schedule and construction zones, and he leaves Sam in Rachel's company.
That's the beginning.
In all honesty, Dean's a bit pissed when Sam tells him he'll move out of their crappy little two-room-apartment to move in with Rachel. But he doesn't say anything; they've been together for a year or so, Dean guesses it's the logical next step for them. He doesn't want to pin Sam down, keep him from living his life; sharing a place was ever meant as a long-term solution, anyway, but he dreads the thought of living alone.
Turns out, he doesn't have to. In the end, he spends more time over at their place than he does at home.
When the phone rings in the middle of the night, Dean's first instinct is to pretty much freeze with fear. That can't mean anything good. He half expects it to be Rachel, telling him something happened to Sam, and that might get reflected in his tone when he flips the phone open and pushes out a weary, “Everything okay?”
But it's not Rachel, it's Sam, and he doesn't sound like there's anything wrong with him. In fact, he sounds downright ecstatic. “Yeah. I'm fine. We're fine. Rachel just told me, uh. Dean, how would you feel about becoming an uncle?”
Sam doesn't tell him about the name beforehand, and so Dean almost has a friggin' heart attack when he sees the label on the crib that says 'Deanna Hawthorne'. He glares at Sam, who stands beside him with a smile on his face so big and bright that it outshines the deep shadows under his eyes.
It's been a long day; Deanna took her time. Dean knows, he's been right there, waiting outside the room for the whole of it.
“You're such a sap,” he says. “I'm embarrassed to be related to you.”
Sam doesn't reply, just grins even wider.
Every now and then, Dean tries to date. There's two ways that usually goes: with a fuck that leads to some halfhearted thing that barely deserves to be called relationship, or a promise to call him again which isn't kept.
His looks still get him some action, but it seems women aren't too impressed with what they see after they started scratching at the pretty surface. He can't blame them.
Only once, when he's forty-two, things take a different turn. Her name is Megan, she's blonde and curvy and single mom to a pre-teen called Jason, and she sticks around for almost two years. It ends in tears and broken dishes, and after she told him to get the fuck out of her life he doesn't bother going home, drives straight to his brother's house.
And Sam doesn't have to be told. He takes one look at Dean, tells Rachel he's going to cook lasagna – the only thing he ever learnt to make from scratch – and allows Deanna to stay up late. She falls asleep curled up next to Dean, halfway into a Shrek rerun.
So yeah, whatever. He doesn't need a girl of his own to have a family.
As a pre-schooler, Deanna gets really scared of monsters. It's probably a phase, perfectly normal, but Dean can see it freaks Sam out.
They made sure, years ago, that there weren't any left, no monsters in anyone's closet; there's nothing that might indicate the bane has been lifted, or even grown weaker. On more than one occasion, Dean's witness to Sam assuring her that no, definitely, he's one-hundred percent sure that there's nothing waiting for her in the dark. Deanna's supposed to grow up in a world without demons and ghosts and creatures – it's what made them both settle down in the first place. For her, they're going to be things she only knows from books and movies and fairytales.
“But, what if?” Sam asks him one evening over an after-work beer, after he brought her to bed; he's a grown man in his late thirties, but he still manages to look as scared and lost as his daughter just did. “What if it stops working?”
Dean sets his bottle aside, glances up the stairs to where his niece is sound asleep. “Then we'll do it again. Nothing's going to take this away from you, I swear, not this time.”
The first time Deanna shows interest in the opposite gender, she's ten. One morning over breakfast, before soccer practice, she casually announces that there's a boy she likes and that she's going to marry him.
Later, Dean has to talk Sam out of murdering the kid as a precaution; Sam's bitchface when Dean tells him to “calm down and let it run its course” is legendary.
Secretly, though, Dean gets it. She might not be his kid, but she's his family, and if Sam'd feel the need to commit bloody murder to protect her? Dean'd be right there with him.
Deanna learns how to drive in Dean's car. She asks for it; the Impala is a passion they share, and by the time she's a teenager Dean long since accepted the fact that he can't deny her anything.
That doesn't keep him from theatrically gripping at the dashboard every time she takes a sharp turn, though, or from chewing her out when she shifts gears too abruptly.
She drives like Sam, and Dean can't resist telling her so.
“Oh man, Dean,” she replies, and the way she draws out his name and rolls her eyes is just like Sam, too.
When he's sixty-two, Sam gets sick. Really sick. He spends weeks on end in the hospital, until they diagnose him with a disease that will eventually render his liver useless.
They give him six months, but Sam's a stubborn son of a bitch and holds out for two more years. Dean's alone with him when his function readings take a dramatic turn for the worse – Rachel and Sam split almost six years ago, and Deanna has a job and a husband and Sam insisted early on that she shouldn't waste hours on end in the hospital with her dying father.
The doctor who checks Sam over tells them that this is it, mere hours left now.
Before Dean leaves the room to call Deanna – no cell phones allowed inside the ward – Sam takes hold of his wrist. “Wait. Promise me something.”
Dean stops dead, swallows. “Yeah, of course. Anything.”
“Look after her, will you? My little girl. Gotta be there for her.”
The plans Dean had for after Sam's death were kind of short-termed, and he knows when he's been led into a trap. But he's not selfish enough to deny his brother his dying wish. “Cross my heart, Sammy.”