You know that story where...
...a brother visits his sister and finds out she's in a relationship with her apparently female roommate. The two big reveals besides the eventual incest content are that the roommate is a transsexual male-to-female, and that the sister is some kind of sex worker. The brother first saw them going at it through the skylight of one of the rooms of the apartment.
Any takers? It was an excellent story, and I'd love to be able to read it again.