Welcome! Thank you for your question.
If your mother is competent then you cannot force her to do something against her will. If your mother is not mentally competent then you can file a petition for adjudication
of your mother incompetent and request that the court appoint her a guardian. The guardian can force removal of your mother from this situation. While guardianship is an option to take control it does pit you against your mother in this incompetency proceeding.
Another option is to report your sister's actions with the local adult protective services agency in her area. The adult protective services office can investigate abuse, whether your mother is competent or not. If the agent finds abuse then the agent can force a change in the situation. It is possible that they can even refer the case to the local prosecutor for violation of elder abuse prosecution.
I cannot provide you with legal advise. I have provided you with information about the law related to your question. My answer, and any information that you find online, should not take the place of having a consultation with a lawyer in your area to advise you regarding your specific issues.
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