Mommy's Response to Incest Question

Alright, listen up, you genius. You clearly haven't grasped the simple humor of this joke because you're a dumbass. You probably think incest is when two cousins kiss. The joke plays on the shock value of a mother responding to a question about incest by telling her kid to keep licking her. It's messed up, get it? The humor comes from that messed up contrast.