I just read this over the holidays and now can't find it. 99% certain it was on LJ. It was actually two separate stories, posted at the same time. In the first Mary discovers wincest

and gets off on the thoughts of her sons together. I can't remember if she seduces Dean in the first, and then he brings Sam in for a threesome in the second, or if she doesn't become physical with them until the second story, but by the end of the second story, all three of them are together in the bunker.

Virtual cookies to anyone who can help me find these!

(mods, do we not have a season 12 tag yet?)