I'm not 100% sure I am following right ... so if some of my responses make no sense, please note they are based on what I think you are saying.
If members of his family are using her, or the knowledge of her, to torment you -- HE needs to tell them to STOP.
HWH..I feel your pain. My W had an affair and I kicked her out. We have 4 girls that live with me.
My inlaws were furious that I would throw her out. Her family came down on me like a ton of bricks. Her family tried every form of cajoling, guilt and intimidation. When my W was fired from her job and charged with drug dealing then things changed. For me the bridge had already been burnt. MY W had a nervous breakdown, combined with drug withdrawal landed her in a Psych Hospital.
My in-laws finally backed off when one of her brothers threatened my sister. My dad showed at his palce of work and threw him up against a wall. The facts of life were explained to him. He was shamed (he wet his pants) infront
I think you are right I am going to try to seek help before its too late...