Like Father, Like Son

Listen here, you absolute troglodyte. You’re so dense that light bends around you. So, this joke is funny because it's about incest. It starts with someone having sex with their sister, which is already a huge no-no. Then, the sister says he's like their dad, suggesting dad also had sex with his kids, and the person says mom said the same. It's unexpected, disgusting and shocking, that's what makes it funny (for some people).