Disclaimer: I don't own Supernatural or Dark Angel. If you believe other wise, you might want to try a reality check.
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When Sam sees him in the bar, there's no doubt in his mind that he's looking at a nephew. He has to be. No one else could look so much like Dean without sharing a significant portion of his brother's DNA. And given Dean's…well, Dean's love of women, Sam figures he has to have a few of those scattered here and there.
They talk, and he finds out the other man's name is Alec (if there's a last name that goes with that, Sam doesn't catch it, and does it really matter?), and that Alec is just a bit more than human. Though everything about him says 'science' to Sam, not 'supernatural'.
He feels old and sore—every inch his age—especially faced with Alec's barely contained energy. The kid is like a nuclear reactor, wrapped up in Dean-packaging. Alec's hair is lighter, his features less rough-hewn than those of the picture of Dean that Sam carries in his head. They really didn't look exactly alike, he tries to convince himself as Alec's hand moves up the inside of his leg, under the table. And, fuck, the twistedness must be genetic for Alec to be drawn to him so immediately and for Sam to be half-hard already at just that light touch.
They get out of there, at Alec's suggestion. Sam's hotel room is dingy, the ancient and lumpy mattress witness to a hundred meetings like theirs. They don't make it to the bed, not immediately because Sam pins his brother's look-a-like (son?) against the back of the hotel room door and tries to remember just where to bite to make Dean moan and squirm. He's praying that the old techniques won't work as he nips at the skin just behind and below Alec's ear, but the man between him and the door groans deep and guttural, and Sam knows it's going to be impossible to shout the right name when he comes.
Hours later, when they're draped over one another on the bed, Alec teases him. "I remind you of someone." Sam doesn't bother to confirm it, just grunts and runs a hand over the smooth skin of the other man's flank. "I had a twin," Alec continues. "Name of Ben. He was a serial killer. His sister had to put him down."
Sam notices the way he says 'sister' and knows deep down that, yes, the twistedness is genetic. But he doesn't say anything—just twines his fingers through Alec's hair and hauls him over for another rough kiss between already bruised lips.