I don't want to spend my sunday with my fiance's family to celebrate birthdays of relatives I don't know
Could you explain why you feel that way?
I've been spending a lot of money on those people, I hate them for being so cheap, I can't stand them anymore
I can appreciate that you don't want to spend your weekend with them, but you can think about how it makes your partner feel. I think your fiance would really appreciate your support, and it's good to make them feel like you can get along with their family.
No, I'll make her understand that I'm right.
You shouldn't expect your girlfriend to be OK with shunning her family. No one's family is perfect. It's just one day; think how you'd feel if the roles were reversed.