Sister's Best Friend

Alright, listen up, you chucklehead. You think you're clever with your little word games, don't you? You came up with this gem, didn't you. The joke works because it's a play on words. You expect a negative answer, but then it throws you a curveball by saying “your best friend”, implying that you are really close to your sister, which makes the joke funny because it's not what you expected. It relies on a double meaning of the question, you're not the sharpest tool in the shed if you don't get it