How do help my sis? She’s not helpless, but she does need help.
By being you. By being her big brother. By trying to find answers to difficult situations. By loving her. By keeping yourself healthy too.
You have come so far. You both have. You are doing what you can to protect her while still trying to be there for your brother. Just as she has had to lean on your parents in the past, you too can lean on your parents now. Neither one you need to be going through any of this alone. You guys make an awesome team but sometimes a team needs a manager. None of this is going to get resolved overnight. It will take time. It may even get worse before it gets better. But I totally believe that the love you guys share can and will conquer whatever life throws at you.
J my ex wife has borderline, and I suspect that my brothers wife has it or at least borderline traits. Its a serious mental illness kind of similar to bipolar. Its really more than a “personality disorder” I thought that I had some borderline traits on top of my schiz and bipolar - my therapist said no. There is a therapy treatment - DBT to help with the symptoms.
Stand up to your brother. Explain to him that as long as he feels the need to be a brat, then he is not welcome to be in your life.
Take off the kid gloves if you want him to change. He is not a child and will come around as long as he is not getting what he wants. It will take however long it requires to learn he can’t act out and hurt people just because he is angry. Everyone connected with him will be for the better.
Don’t let him play on your sympathies, he can’t say he isn’t responsible for his behavior. He is.
Help him be the person people want to be around, otherwise he is destined for a long and lonely life.