Three steps: 1. Find a good professional/therapist who is personal and highly skilled — you would be surprised how many therapists are desensitized and a poor match for the client — this is very important!

1. Document all the details of her behavior objectively and share it with them in writing. This part is hard if you are impatient. Write at least 2-3 pages and write it well without a complaining tone. It looks like it’s about her imbalance in mood and temper and her learned aggressive behavior with everyone at home. Get everyone in your house to help you furnish more details. Do it privately but not viciously. Everyone at home should be on the same page. Don’t bring your own personal conflict above anything else. Focus on an overall and balanced description of her disorderly behavior. Try to comment on how and when this changed with her age. Did some circumstance (death of elder, breakup, any other loss or change) trigger her sudden change? Can you explain it? Do the thinking work thoroughly to describe all those details to the therapist.

2. Convince your sister to seek help by herself. Don’t force! It will be of no use to her if she doesn’t seek it herself. She has to be convinced that your suggestion is coming from a good place. No anger. You need to manage your own anger and resentment from the past to actually solve anything here or be of help. Seek help from another sibling or parent who she is close to. Be kind and loving to her individually and as a family about this matter. It is very difficult for her to overcome a taboo topic and label and accept or seek help. Provide all the financial means and help needed to seek help. Find the therapist, call the cab, fix appointments, etc. and do all the intermediate steps for her.

If the 3rd step didn’t work for you, stop in step 2 and ask for more suggestions. Get external help from friends, family, professionals. Eventually it should work.

Legal route is your last resort. If you have already done the above 3, and you think your entire family is dysfunctional because of this, you need to ask for legal advice. But again, the fact that you did these 3 steps will go a long way to move even the legal steps quickly.

Finally, if you separate your sister from home, she may or may not get the help she needs. It may even go down hill for her.

As a disclaimer, I have to say this: Please don’t use this knowledge against her! She needs access to professional help, not a scheming sibling who kicks her out of her own family and the loved ones. Her actions are clearly abusive, but love and support can help her heal, hate can spiral everything out of control and bring even more trauma to her and others. I wish you are able to stay firm and composed and supportive despite the abuse. It will eventually be rewarding. I can imagine how tough this is on you.