Your brother can date whoever he wants to date. That doesn't mean you have to kowtow your behavior in his favor. Ignore her and grimace whenever you have to look at her.
Eventually he will get secondhand wind that you don't like her. He will either stop coming around or will beg you to not be a douche. Negotiate your terms appropriately.
Eventually he will get secondhand wind that you don't like her. He will either stop coming around or will beg you to not be a douche. Negotiate your terms appropriately.