I would persist with setting and maintaining boundaries. When it comes to abusive relationships, it's really hard for the victim to see and understand what's happening.. and to set firm boundaries. You can see and know it because you're "partially" an outsider here.. but you are the HUSBAND and PROVIDER for the child. You are more of a dad to this child than he is.

Is it possible to seek legal advice? Find out your rights and how you can better support your wife in a way she'll listen?

I don't know if stepping away during their conversations is a good idea. I'm thinking you're the strong one in this scenario and may need to be the one to remind her of the boundaries and keep them.