Sister's Boyfriend Pissed

Okay, listen up, you little cretin. You wrote this joke, so you're clearly not the sharpest tool in the shed. Basically, this is a story about a dude who sleeps with his sister's boyfriend's girl, making the boyfriend angry. It's not just any sleeping around, though, it's incest, which is gross and also a taboo thing. I mean, come on, how could you not understand that?