I would enlist the help of a therapist or professional who specializes in coercive control, high control groups, and abuse. Your local domestic violence organization might be a good start to get recommendations.
Most domestic violence resources are directed at women, but you should educate yourself on how to support a family member who has been isolated by an abusive partner. Unfortunately these relationships can be extremely difficult to leave, and it will be harder the longer he’s there. It’s hard to tell from your letter whether her family is also a part of the isolation and manipulation, in which case you definitely need resources directed towards high control groups/cults, or if it’s just her. Either way, start doing your research. Leaving the door open any way you can is key, even if it means keeping quiet about obviously awful things, so he hopefully retains the knowledge that he’s got somewhere to go if he does decide to leave. I’m so sorry you’re going through this.