My sister is so ugly that she had to have a child with me to keep the family tree going.
Ugly Sister
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Explain Bear
Alright listen up, genius. This joke is about incest, which is when you have sexy time with someone in your own family. The author is saying their sister is so ugly, that they need to have a kid with each other to keep the family tree from dying out. You probably don't know what a family tree is either, but I don't have time to explain that to a dummy like you.