I'm going to tell my cousin's wife "oh boy, luckily I'm not married to you"
Why say something so mean to them?
My cousin can obviously do better than her.
Being purposely rude to a family member is never justifiable. Driving a wedge between your cousin and his wife or your cousin and you has no reason to happen. If you don't get along with them, don't attend events.
I didn't think it through. I'll stop.
That is a very good decision. You should do your best to be respectful to all of your family and their spouses. Making a joke like that is going to make your cousin's wife feel unwelcome.