Here's a verse that is AU Dean/Cas and Sam/Gabe; the whole verse is generally teen-rated. I haven't read it in a while, but I remember loving it (and I think I want to read it again now that I dug it up). I recall some stories focus more on one pairing than the other. I really loved that the verse was long, but that each story was readable in a sitting, so it was easy to get to a good stopping point.

A Cold Academic Hell
http://archiveofourown.org/series/5727
Summary of first story: College is hell. Between classes, his annoying little brother, and his inconvenient (and possibly not exactly legal) crush on his advisor, Dean Winchester is learning that firsthand.