There is a ubiquitous taboo against incest. Cultures vary in closeness of kinship for the taboo, but parent-offspring sex or between siblings is universally condemned; though brother-sister marriages have been practiced throughout recorded history with some regularity.
In the United States, 30% of girls and 17% of boys are sexually abused by a family member. Almost half of child rapes are by someone in the family.
The consequences for the victim are life changing. Sexually abused youth are twice as likely to have criminal records, mental health problems, and commit suicide. 95% of the teen prostitutes in the US were first raped by dad.