I mean it's kind of weird I guess...but I think this type of thing might happen btw. sisters more than people talk about. Obviously they learned a lot of this behavior from their mom....not the specific acts, but their sexual openness in general. I honestly think it's kind of cool, but very different from any family I have ever known. Sooooo the real issue is are you comfortable with being in a relationship with a woman who has these types of family dynamics going on? I am sure the mom (your GF knows). If you stay with your GF for a long time...her family will become yours. I bet they will all (3) ask you to sleep with them!