He's not my family!

I got a quickie! My cousin is planning her husband a huge surprise 40th birthday party. I think it's a great idea. However, she has refused to ask for help from his family. Instead she has been asking our family members to do certain things. Now I like him, I have to put that out there first. He's a great guy, but how are you going to ask your side of the family to bring dishes, and just allow his side to "just show up". That's some bull that I'm not feeling. And he's said on more than one occasion how we are "insert wife's name here family, not his.” So shouldn’t his wife be going to HIS family asking for contributions? Or am I missing something?