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It sounds like your brother has married a woman suffering from Borderline Personality Disorder
. Let me give you a website that will help start your education process on what is wrong with her and how to handle her. I highly recommend the book, "Walking on Eggshells" by Randi Kreger. It has tools that work when dealing with BPDs, and helps you set boundaries and limits so that you can be sane.
The main thing to know is that you can't make this woman feel better. NOTHING you do will work, as the problem is inside of her, and BPD's have an extremely limited capacity for insight. It takes lots of hard work for people with this disorder to recover, and motivated ones do, but the vast majority spend their lives stirring up tornados in other people's lives and never look back. She will do whatever it takes to spit up the family so that she can have total control over your brother. Isolating him is part of the plan, but she probably has no awareness of the pain she suffers. BPDs go from second to second with no account of history other than what they feel in the moment, and that always is in flux.
You're not crazy!BPD Central
My best to you.
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