How to Deal With a Separated Family

My father-in-law has been dating his current girlfriend for years. He feels she should be allowed her as a plus one to guess what the wedding will look like. My fiancee and i have never asked any questions about this relationship. We are going to live a healthy life either together or you know ratz fishelam apart it doesn't matter we are going to have a healthy relationship even if there's conflict in your family do you know what i mean  your parents have some bullshit that i don't want vibing onto our new shit at this wedding"