I cannot recall the title or author of this story. Here's what I remember (as I believe Sam was underage in the story, I will put it below the cut):

It was on LiveJournal and probably around 5k, give or take. Dean and Sam (Sam is probably underage) are at what I believe was a high school party with Dean's friends. Sam gets drunk and is mostly passed out on a couch when Dean finds him. Dean takes advantage of Sam's condition and rapes him. Sam never quite wakes up. In the morning, Dean makes Sam feel guilty because he tells his little brother he had to watch out for him and protect him. Sam has no idea what Dean did while he was passed out and is grateful Dean "took care of him".

Thanks for the help!

Found by a helpful soul on Tumblr. Just For Me by goandgetthegun on Ao3.