A pair of siblings ( brother and sister or two sisters ), twins or otherwise, around the age of 20, are close as siblings can be. In fact, they're considerably closer than most, they think alike and share most their secrets. While not completely shameless around each other, they don't mind cuddling in ways that might border on inappropriate, and have shared more than one kiss. Still, nothing sexual has ever happened between them.
The way I see it, that's about to change. This can either be a complete RP, or a simple one shot. Either way, the scene I have in mind starting it all off is one with the two of them on the couch, watching a movie. More or less out of the blue, one of them asks the other if they would like to give/receive oral sex, and the other accepts. Cue either an evening of experimentation, or a long relationship.
PM me if interested!