The act of dating your sibling.
Mom: Where are you and your brother going?
Mary Sue: Billy Bob and I are going incestating!
Billy Bob: I'm gonna' get me some of that good sister lovin'.
Dad: Billy Bob, you know I shoot at anyone who tries to date yur sister, but since you are my boy I guess I'll let it slide tonight.
Mom: Have a good time and practice safe sex you silly kids.
Dad (to Mom): Come on and watch TV with me, it's our favorite show, "All in the Family".