Have an ex that treated you like complete sh*t? Want to get sweet sweet revenge? Does your ex have a hot sibling? Have you figured out where I'm going with this?
Nothing says revenge quite like f*cking your ex's sibling. Sure, it may make you seem bitter and crazy, but if they cheated on you and screwed you over, then why the hell not?
But some people do the dirty with an ex's sibling just because they think it would be fun. They treat it like some sort of a sick game and feel no remorse. Hell, they are even a little proud.