I'm going to go off on my cousin for being a completely tactless human being
I am sorry to hear you are having problems in your family. Is there some kind of history behind that?
I haven't been able to work with him for the last month because he's been so cold and indifferent to me.
Standing up for yourself can be tough, especially with a family. But, it sounds like you know you have to talk to your cousin about his behavior. Explain why things are difficult and make sure they understand how much it is affecting you.
I didn't want to talk to him because he's a jerk. I should back down, but he needs to understand his actions and hopefully my family will stop letting him act like that.
I think you are doing the right thing. People are expected to stand up for themselves and what is right.