What naturally keeps brothers and sisters from having sex with each other?

If there were no human laws and nature could just take it's course, would teen siblings end up having sex, because their hormones are going crazy at that time and there's a person of the opposite sex living conveniently in the same house? Or is there something that happens in nature that somehow keeps animals and humans from having sex with each other?What naturally keeps brothers and sisters from having sex with each other?That's a really good question. I think it is the actual formation of family units, with parents that stay together and raise children, which causes the prohibitions about incest. The father and the mother encourage their children to form bonds outside of the family, not only because of social mores, but because of some kind of desire to keep their family separate and controlled. I don't know how it developed but I consider it to be a sign of civilization. Incest is sad and disturbing.What naturally keeps brothers and sisters from having sex with each other?Though this is speculation on my part, it would appear that those of us who were not inborn survived better, due to a lack of deformities and mental health issues. Dogs can still be inborn, but in not sure that it happens often with their siblings, mainly with their grand parents.What naturally keeps brothers and sisters from having sex with each other?I hated my sister.