My sister does not care about my Mom but wants her money and will stop at nothing to get it. Any advice?

Xmas spent with Mom. Sister not interested. When banks open she brought solicitor with witnesses to declare her mentally unfit. She got every penny of my father's money and is determined to get more money from my mother. Teresa, my sister, is extremely wealthy from my father's money and lives in a luxury detached house, drives an expensive sports car and wears designer clothes. She will not help with care for my mother, unless it's to stand down carers/damage my mother's right to care and isolate her. She refused to pay for my mother to have TV when she was in hospital, despite having a large income (myself and my husband paid although we have little money), wanted her to die in hospital (but i fought to get her out) has no interest in my mother's welfare and couldn't even be bothered to give her a present or wish her Happy Christmas. I spent Christmas with my mother, cooked for her, bought her lots of presents and she had a lovely time. 2 days after I left, my sister arrived with her husband and her son and brought a solicitor to the house - forcing my mother to sign paperwork. My mother doesn't want to remember what happened, saying only that it was horrible, she was scared and they didn't get her any food. (She called me to say they hadn't bought her food so I since arranged for a meals delivery company to bring more) I believe that during their arrival with the solicitor and her 'witnesses', my sister declared my mother mentally incapable so that she can control her money. i have arranged for the carers my mother requested - but if my sister now has power of attorney over my mother she will not pay them and I'm scared this will kill her. My sister Teresa wants my mother to die.
(In 1999, when I visited my father after she had secured all his money he was in sheltered housing for poor people, having previously sold a villa in Spain (my sister Teresa and her husband took all the money). When I visited my father in the sheltered housing, he was totally blind and the studio room was infested with ants. Teresa and her husband did not arrange any social care. (As soon as I saw the terrible state he was living in I organised care for my father.) Unfortunately, due to being left in poverty, he did not survive long. My sister Teresa wants the same end for my mother so she can have her money. What can i do to save her??? Please help. My mother will not survive if my sister Teresa has her way. Thank you.
A very frightened and concerned daughter.