My brother's girlfriend is so obnoxious that I feel like calling her an overbearing cunt.
That is some harsh language. What about her bothers you so much?
Everything. I just need to say something really bad about her.
Overbearing people are so draining for you, but this name calling seems childish. Be more adult about it, talk to your brother so he can fix the situation himself. Bad language and words can lead to more family strife that gets difficult to remedy by anyone.
My brother might not listen. I need to have control over the situation. I'm going to give her a piece of my mind.
You're right that your brother might not listen, but you should still try talking to him about this issue. If you just suddenly insult his girlfriend, he will be very upset with you and not want to speak with you. If you explain to him how you feel about her, it's likely he will understand and try to make things better.