You can't threaten a man with the death of his sex life

I'm always disturbed by how my parents and some of my family members say things like, "I will disown you if you ever do something with your sibling." Where do these people get off threatening to cut off entire limbs of the family? Like sorry, but my sister is so hot I would totally do it with her. Always have. Always will. Everyone in our family is just waiting for us to do it. They don't get to control that.