A daughter's confession

Listen here, buttercup. The joke is funny because the mom tells Abbie that she can tell her anything. When Abbie reveals that she had sex with the dad, the mom gets angry and tells her to go die in a hole. This is funny because the mom's reaction is unexpected. Also, you're not very smart, kinda like the person who wrote the joke. I bet you still think the Earth is flat, huh?