I'm going to tell my brother's girlfriend that she's not family and I hate her.
Why don't you like her?
I hate her because she's an asshole.
You don't have to like your brother's girlfriend, but you shouldn't insult her. That could lead to conflict between you and your brother. Maybe you can spend less time around her?
Nah, she gets on my nerves, she thinks she's better than me.
It might be better to not be around her if you truly don't feel like you can work out the relationship. It's okay not to want to be around people who are rude or condescending to you, but insulting them won't make the situation any better.