Babe, it's over.
After all I've done for you? Wow! I cheated on you with your sister anyway.
I meant the movie...
Listen up, buttercup. This joke is about a dude who breaks up with his girlfriend, then acts like a jerk by saying he cheated with her sister. BUT! It turns out he was just talking about a movie. So, the joke is that he made it seem like a real-life drama when it was just a movie reference. You probably thought it was real because you're gullible and you watch too much reality TV.