I'm going to flirt with my boyfriend's brother and show it to him so he knows I can replace him anytime.
Does your boyfriend know about this yet?
I think he knows, but can't do anything about it (he's currently non-English speaker)
It's horrible to make your bf jealous on purpose. You shouldn't be trying to hurt him.
I'm going to flirt with his fucking brother, what's he going to do about it?
He is going to get upset. If you want to have a good steady relationship than trying to make your boyfriend upset on purpose is not a good idea. You should be trying to make one another more happy instead.