Pat,

This sounds like a bigger thing than one person really can handle. I'd get adult protective services involved, since this seems suspicious to you, the son-in-law has acted abusively towards you in front of your client (and one can imagine that he probably also is abusive to his family members, including your client), and you suspect that he is taking advantage of her and using her resources inappropriately. What he is doing is wrong, and it can't be allowed to continue, but it will probably take bigger guns in order to do anything about it. You know, you could also contact law enforcement about his threats to you. And he can't fire you if you aren't working for him, right?

Good luck to you!