Just posted this mini fic on tumblr, thought I'd put it here as well. Just a short little Sastiel/Destiel fic.

Disclaimer- Don't own supernatural

Sam doesn't know why he followed Dean and Cas. He knew what he would find, why they needed some "privacy." Sam chuckles despite the raging emotion in his heart; Dean thinks he's being subtle. Yeah right…far from it. He knows his brother better than anyone, as if he could hide this from him.

Sam sits in a spot he knows they won't see, but he can see them all too clearly. He can see how Dean's shoulders aren't so tense, how his eyes don't seem so dark. He looks so much brighter now that Cas is back. Sam manages smiles at his brother's happiness, he deserves it.

His smile falls when his gaze travels to Cas. Castiel doesn't smile much, but now he's practically glowing. His smile is bright and wide, he can see the sparkling blue in his eyes from here. He has to look away for a moment, the hurt and jealousy clenching his heart. Castiel has never looked at him that way.

He tries to fight it off, he shouldn't feel this way. It's so unfair that both he and Dean have to fall in love with the same angel. Dean deserves him, why couldn't he fall for someone, anyone else?

The last time he remembers feeling this way is with Jessica, he was admittedly a little possessive over her. He loved her deeply, but this? This is so different...yet exactly the same. He loves the angel, at first he was confused by it. Sam and Castiel aren't exactly BFFs like he is with Dean. But still, in the time he's spent with Cas, he can't not feel this way. He knows Castiel feels something for him. Definitely strong friendship if nothing else. But Cas has made it all too clear that he knows how Sam feels.

His gaze travels back to the reunited couple, ignoring the pain it brings him. He tries to focus on the fact that Castiel is happy with Dean, that he made the right choice. Deep, deep down, Sam knows it's better this way. It still crushes him, knowing that he will have to look Dean in the eyes and see the happiness there. Even worse, looking into Castiel's eyes and knowing that he isn't good enough. It would be so much easier of Cas didn't know, if Cas couldn't read his mind. If Sam could just lie, he might be able to look both Cas and Dean in the eye and say "I'm real happy for you."

Then he'd tease Dean about how he doesn't have to keep overcompensating anymore. He'd taunt him for months until Dean would just flip on him one night and he'd laugh his ass off about his brother falling in love with an angel.

But knowing that Cas knows, he just can't. He's not strong enough and if Dean ever found out…he doesn't want to imagine.

Cas and Dean kiss, their eyes falling close and bodies pressing together. Sam takes this as his cue to leave, partly because he needs to get his emotions under control and also he feels like a sicko creeping in on his brother. But just as he's getting up, Cas pauses, his eyes opening and he looks Sam straight in the eyes. Sam sees guilt and compassion wash over his features. He frowns and nods minutely. Sam tries to react, give him a reassuring smile, but it falls flat. Castiel shouldn't feel guilty for this, he already has enough guilt to deal with.

Dean notices his distraction and turns to look, but Cas stops him by placing his hands on Deans face and pulling him back, crushing his lips to Deans.

Sam sighs heavily, forcing himself to turn arway. It doesn't stop him from hearing them even as he walks quietly away.

He hears Dean ask, "What's wrong?"

"Just...nothing Dean. I'm glad to be back."

Despite this outcome—however painful it is— Sam is glad Cas is back too.

Thanks for reading!