1) girl!Sam. From Dean's POV, overnight Sam gets turned into a girl. It starts out in the morning after, with Sam waking Dean up, who responds with something like "Hey, baby, you still here?" Sam calls Dean a pig and they go on from there. I think this is the same story where they go to buy Sam clothes at the mall, and Dean is rather over protective and keeps envisioning large sweaters and jackets to hide Sam's new body from the starring guys. Sam does end up getting a jacket, but it's one of those short ones that stops not long below the breasts. Ends up Wincest.
ETA: It's
joyfulgirl41's
Opportunistic Lover.
2) girl!Dean. Dean is cursed by an Incubus/Succubus, and ends up a rather horny girl. Sam figures out what cursed Dean when they go to a club and even more people that usual come on rather heavily to Dean. Pretty sure it's Wincest.
ETA: On second thought, this could also apply to number one. Still think there might be one where Dean is cursed by an Incubus/Succubus.
3) girl!Dean. Cursed, I think, by a witch this time. Gets his period almost immediately, and ends up sending Sam in to get the supplies. Sam gets pads, nice safe pads, after he reads the tampon box and is surprised how they are used. Curse ends when Dean realizes something like there is no way a guy can really understand a girl, unless he has been one. Might actually
not be Wincest.