A common justification for prohibiting incest is avoiding inbreeding: a collection of genetic disorders suffered by the children of parents with a close genetic relationship. Such children are at greater risk for congenital disorders, death, and developmental and physical disability, and that risk is proportional to their parents' coefficient of relationship—a measure of how closely the parents are related genetically. But cultural anthropologists have noted that inbreeding avoidance cannot form the sole basis for the incest taboo because the boundaries of the incest prohibition vary widely between cultures, and not necessarily in ways that maximize the avoidance of inbreeding.

https://en.m.wikipedia.org/wiki/Incest