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A man who fathers a kid with his own sister. He's the kid's dad, brother, and uncle all at once. Basically a human mess.
My cousin is a fabruncle. He knocked up his sister and now he's my dad and my uncle.
My uncle's kid is my cousin. But also my brother. He's a total fabruncle.
My mom's brother had a kid with my mom. That kid is my brother and my cousin. My uncle is a fabruncle.