I'm going to start a fight with my friend's sister's boyfriend
Why would you want to start a fight with him?
Because I hate my friend's sister's relationship.
Even if you don't like him, you shouldn't start a fight with him. You should try non-violent ways of getting things done.
I'm gonna warn him that I'm gonna mess him up.
I just think it's wrong that you would even start a fight with him. Why not just ignore him instead?