I don't want to go to my boyfriend's brother's birthday party.
Why don't you want to go to his party?
I don't get along with his family and these family gatherings are difficult.
You should go to your boyfriend's brother's birthday party. It's important to make an effort to get to know the family and feel comfortable with them.
They're all hypocrites and I don't like what they talk about. I'm going to be very uncomfortable at the party.
Your boyfriend will appreciate the effort you're making to be part of his family. Try to go and have a good time. Don't let others affect you mood.