Yeah, you're right. Maybe I didn't want to admit to myself that this wouldn't be such a problem if I were thinking purely in consequentialist terms, seeing the disruption of social dynamics and inbred children it would probably cause. However, I actually don't know how much it would change the amount of sex siblings have, it could be that those who want to do it are already doing it at the moment. "The disruption of social dynamics" argument has been given against homosexuality, but I don't think it holds weight as much because homosexual relationships resemble more heterosexual relationships than relationships between siblings resemble homosexual relationships.