Read this!
I quote...
"Psychiatrists claim people usually cannot remain good friends after they stop having sexual intercourse together. If they don't remain lovers, they must become very distant from each other. In a close situation such as attending the same school or the same church, this "distant" relationship can conflict with being near each other in classes. The two former lovers can develop a "hate" relationship as a way of maintaining the "distant" relationship.
An example of this hate after intercourse is Israel's prince Amnon and his affair with his half sister Tamar. After sex, "his love turned to hate and now he hated her more than he had loved her." -- 1 Samuel 13:15 LB."
Of course the fact that their relationship became dysfunctional after sex had, of course, nothing to do with the fact that they were actually brother and sister.