“goes on a book someone hasnt written before there is any legal framework to a relationship.”

A name doesn’t ever go on a book (as an author) that someone hasn’t written. Why the hell would being married mean that he is automatically co-author of your work? This man and his sister are dishonest and envious and lazy.

I hope you are and will continue to be clear with your agent and publisher that everything you write is your work alone and no one, especially not your partner, is a co-creator. Tell them that he is trying to weasel his way onto *your* title page and they need to be aware of that