I'm going to date my ex's sister
Why specifically her sister?
I want to get revenge on my ex
You should never get in a relationship just for revenge. Just move on and leave your ex alone.
It's a win-win, I get the girl and tick my ex off.
I believe that you shouldn't pursue such a relationship if it's built off of spite. Don't you think that's unhealthy for everyone, because your motives aren't in good faith? Why not completely move on from your ex, and find another fish in the sea?