He knows. And he's pretty certain Dean knows he knows.
Sam noticed early on the intensity of his brother's relationship with Castiel. It wasn't that surprising, what with the whole "gripped-you-tight-and-pulled-you-from-perdition" thing, so Sam never really paid it much attention. And then on the way to Kripke's Hollow after the catastrophe in Ilchester, Sam had been a little too distracted to listen closely as Dean explained the situation he had left back at Chuck's place. But later, when he thought about it, he remembered the strange tenor to his brother's voice, as if it were about to break.
Still, that wouldn't have meant much, except Sam hasn't been sleeping well, lately. Starting the Apocalypse tends to weigh heavy on the conscience. These days he sleeps in fits and starts, drifting off uneasily only to bolt awake a few hours later. Sometimes he'll get up, wash his face, or maybe pace a bit, just something to get this restless energy out. Usually Dean sleeps like a baby--or pretends to--throughout.
Except for this one time, when he woke up to the sound of whispers.
It took him a few seconds to realize people were talking, and then he nearly bolted out of bed before he recognized the sound of Dean and Castiel's voices. Which...okay, weird. He hadn't realized that the angel had started dropping by for social calls in the middle of the night. Alright, it wasn't that uncharacteristic of the angel, but he was surprised that his brother put up with it. Sam opened his eyes.
Dean was sitting up in bed, turned to look at Castiel, who was perched on the side of the bed, back to Sam. They were speaking too softly for Sam to make out more than their voices. Castiel dropped his head, and Dean leaned forward to place his hand on the angel's shoulder, speaking intently. After a moment Castiel lifted his head to look at Dean, his profile sharp in the moonlight. He lifted a hand, hesitated, then cupped it against Dean's cheek.
And...wow. Wow. This was lightyears away from his business, but Sam could not turn away. Normally Sam would have expected Dean to rear back, pull away from the angel, but instead he reached up to cover the hand with his own. And rasped out a single syllable, loud enough for Sam to hear:
"Cas..."
Not my business. Really not my business, Sam thought, and forced himself to roll over.
After that night, he starts noticing things. The way Dean's eyes always flick across Castiel's body when the angel appears. The goofy little smile he gets on his face when Castiel calls. How they always seem to drift towards each other when they're in the same room. Once Sam knows what to look for, it's almost suffocating to be in the same space with them, because Castiel is here not for the Winchesters but for Dean, while Dean...Dean wants to do things that involve Sam not being there. And the air thickens with desire.
It's weird. Weird weird, for a multitude of reasons, and Castiel being male-shaped is only the start. Sam has never seen his brother act like this before, and Castiel, well, is sort of like a massive walking bundle of over-serious bizarreness. And then they waltz around each other like awkward teenagers still figuring out the basics of flirting ettiquette. It should be funny, but it really, really isn't.
Sam can't take it anymore. The next time Castiel shows up, he makes hasty excuses involving nonexistant hunger and a burger place up the road, and gets the hell out. Twenty minutes later he's ensconced in a little secondhand bookstore he noticed yesterday.
Sam likes bookstores. Independent ones, not the crappy impersonal chains. Something about them makes it easier for him to think. Right now, drifting through the Sci-Fi/Fantasy section (which consists primarily of cheesy pulp novels and the usual selection of Heinlein and Asimov), Sam thinks he should be happy for his brother. Dean has fallen in love with someone who returns it fullheartedly, and that's a good thing, right? After all the crap in their lives, after all that misery, Dean deserves a little happiness. It's the end of the world, after all, so it's not like he's going to get another chance.
Except the only thing Sam can think is, Someone else to lose. Because they're all going to die. It's the end of the world, and he and Dean are both signed up to play leading parts--well, vessels to the leading parts--in the play. They have the entire forces of Heaven and Hell gunning for them, and the damn Colt didn't work, and they are all going to die. Hell, the way their luck runs, everyone they know will die. Especially Castiel, who defied Heaven for them--no, for Dean. Castiel will die, and it will wreck Dean, who will also die, and Sam will die, too. Okay, so maybe it won't be in quite that order, but they're all gonna die. Probably painfully, too.
Sam wants his brother to get that happy ending. He wants to see Dean ride off (fly off?) into the sunset with his arm around Cas, and maybe get a letter every Christmas from some godforsaken part of the US, because his brother specializes in finding flyspeck towns in the middle of nowhere. He'd also like a white picket fence in the boringest part of surburbia, complete with the wife and the dog and the 2.5 kids, and for the the Apocalypse to be a little less imnient. And maybe a pony.
Two hours later, he comes back to the motel, burgers in hand. Castiel is gone, and Dean is in the shower, singing Led Zepplin very badly. The sheets on Dean's bed have been smoothed out, which is funny, because Sam is pretty sure they were all rumpled up when he left. A couple of minutes later the bathroom door swings open, quickly swings closed again once Sam is spotted, then swings open once more before Dean emerges, now with a towel wrapped around his waist. "So was the Burger Ranch or whatever it's called any good?"
"I've had better. And worse, I guess...it's passable. Thought you might be hungry when I got back, so I brought you one of their cheeseburgers."
"Yeah? ...What took you so long?" Dean is already digging through the bag, but there's an odd twist to his voice.
"Stopped by a bookstore, browsed a bit...you know."
"Nerd." There's a bite mark on his neck that wasn't there before. Sam doesn't say a thing.