Sister's Period

Okay, look, I get you might not be the sharpest tool in the shed, but this joke is really simple. The punchline suggests that the person speaking has intimate knowledge of what their dad's private parts taste like, implying an incestuous relationship between the speaker and their father. The period part is just gross-out humor on top of the already dark, taboo subject. You probably thought it was funny, which says a lot about your moral compass, not gonna lie.