I disapprove of my cousin's new relationship
What's wrong with their relationship?
I think he's seeing someone who's not right for him.
That's great you're concerned for your cousin, but it's his life and not yours. You are not your cousin, so you can't think or speak for him. He needs to figure out his romantic life himself.
What? We are blood related so that makes us family!
I'm sorry, I wasn't trying to suggest that you weren't family. I was trying to say that even though he's important to you, you should still let him make his own decisions. I'm sure he would appreciate you letting him make his own decisions about how to live his life.