Ok, so the idea is thus:
A man (Or woman, or Futa) meets someone (wherever), and they get to talking. One thing leads to another, and he's asked to come back to her house. She tells him that she shares the house with her sister (Can be Fraternal Twin sister or Older sister), and, they start to have sex, but she has to go to the bathroom before they do, so he's there, sitting nude on her bed, simply waiting...when her sister walks in. And she looks extremely familiar to him! (So, she's possibly someone he dated, or an old friend)..and well..we'll see where it goes from there.
PM me if your interested. We'll need two women (Or men who play women ) for the roles.