So I read this story a long time ago, but I'm pretty sure it was on the SPN Kink Meme. Sam is turned into a werewolf and decides Dean is his mate. From then on he is training Dean to be more submissive and accomodating to him. He times it so that when they stop to eat Dean has to go to the bathroom so Sam orders his good, so eventually even when he doesn't go to the bathroom he still expects Sam to pick what he eats. Then Sam starts dropping his stuff so Dean will pick it up for him. I think towards the end he drugs Dean while they drive to this cabin and he feeds Dean his cum.