In-laws, or out-laws.
So far, I have two brother-in-laws and one sister-in-law. The boys are great, she is... difficult.
I have never come close to having in-laws of my own, but it is something that I think about. The last guy that was somewhat on my radar has family that would be difficult for me to get along with/be accepted by. It was a considerable thing to think about, even when I was just getting to know this guy. Nothing ever happened between us, but he sure was a flirt at first.