I don't want my dad & brother to pay for part of my cousins trip to europe
Why don't you want them paying for your cousin's trip?
Because that's annoying, I'd like to see my cousin pay for herself.
If your family members want to help each other out then it is none of your business. You shouldn't tell people how they can spend their money.
I just don't think it's right.
I understand that you want your cousin to spend their own money, but maybe your dad and brother are just being nice. You should just let them pay and forget about it.