Let's assume there are two participants of legal age. If a father chooses to have sexual relations with his daughter, is it really incest? Think about it, the genetic composure of the daughter is 50% of a man. Half of the daughter is technically the father. It works both ways. Since the daughter is the father, it's technically masturbation if she decides to have sexual relations with the father.
It's only illegal in places that can afford DNA tests in order to enforce it to fine you