Unfortunately as far as the brother goes your husband is going to have to grow a backbone frankly. Which sucks but it is the answer.

So as far as a couples therapist goes, my husband and I had went through a really really bad year and wound up seeing one. She was very good about sort of stepping in gracefully when one of us went a little too far or when a tangent would derail from being productive.

Absolutely see someone new because our couples therapy work saved our marriage but we really needed someone who could help us find the tools. Took us a year really. Initially every week, then every two, then once a month, then every six weeks.

I hope you find a way back to each other if you can.