As a lawyer you know the process to take this decision away from your father and mother. It will require immense fortitude on your part. It will be expensive and a huge time investment. It will likely create an irreparable rift with your mother and perhaps your siblings. It will undoubtedly require your siblings to take sides and likely not all will side with you. That is a huge price to pay.
Only you know if you can live with yourself for not paying that price and enduring the pain. Only you know what your conscience says you owe your father. Only you know whether putting yourself through that meat-grinder will feel like expiation for not standing up to your mother earlier. Only you know if this is the chance to once and forever cut ties with your mother. Only you can predict how you will feel when your mother bars you from ever visiting her or your father again and you have to rely on your siblings for news about your parents.
I have no idea what you should do. I feel for you. This is an excruciatingly difficult position to be in. I do know that it is an honor to be able to share your journey with you. Thank you for confiding in us.