I actually found this fic on this site while I was looking for something else. I swore I bookmarked it but now I cant find it.

Sam went to a witch I guess to find a way to save Dan. She says she'll help but he ha to give something up. He walks through a mirror and into a world where he finds that he and Dean aren't brothers. Dean and Jo are siblings and Sam is at Stanford where he has burn scars from trying to save Jess. He finds out that a demon (Meg) is after him and teams up with Dean(they end up in a relationship)_ . She catches up with them and several people end up dying including Jo, Ash, and Sam. Dean ends up going to the same witch and he has to give something up too. He and Sam end up brothers again.  I could go on but I'm hoping someone knows what I'm talking about. Thanks for your help!