Pairing: girl!Sam/Dean (UST), girl!Sam/OMC, Dean/OFC
Fic Title: All Right, Mr. De Mille
Rating/Warning(s): NC-17. The incest factor here is minimal but still an undercurrent for the story. Also underage teenage hanky panky.
Genre: Coming of age
Why This Must Be Read: This is no simple genderswap fic where a character is turned into a girl as a result of a curse or for kicks or whathaveyou. britomart_is takes a serious look at what it's like to be a teenage girl in the Winchester family with a perfect older brother, a difficult dad, and a whole side of yourself to keep secret from classmates.
Sam has always been my favorite Supernatural character, and her characterization in this is astoundingly good. britomart_is captures every miserable, awkward, disappointing, joyful aspect of being a kid and being a girl. Also, Dean is the quintessential cool brother, worthy of admiration and love. He's helping Sam grow up, and it's SO fascinating how that dynamic changes and remains the same when Sam's a girl.
Honestly, give it a shot. I'm pretty touchy about the incest squick, and I feel like this fic explores those complicated feelings in such a smart, careful, and thoughtful way that it comes across as entirely believable. Not only that, but the language is gorgeous--sparse and perfect.