So you have a character who is a villain. He's kicked puppies, tortured defenseless kittens, and done everything he can to make sure he has thoroughly crossed the Moral Event Horizon. What else can you do to show the audience that he is absolutely evil in every way? Compare Brother-Sister Incest, Parental Incest, Good People Have Good Sex. Contrast Surprise Incest for when the people involved may or may not know they are related, although these kinds of villains usually don't care. Can cross over with Freudian Excuse if they have a weird relationship with their parents.