Please consult a guardianship attorney in the jurisdiction where your mother is living for advice. Guardianship may be the best way to go; Powers of Attorney are suspended when a guardianship is commenced. A guardian would be able to inquire into past actions by your sister. It would help if your siblings joined in the Petition, requesting that you (or one of them) be appointed as guardian of the person and property of your mother. Good luck.
In California there are two things you can do right now. One, call the police and report your sister for elder abuse. Isolating someone from their family is a form of abuse. There may also be financial abuse. But depending on your mom's care, going through $250,000 over a ten year period is not alarming. Care is expensive. Your mom may qualify for some public benefits that might help with her care and reduce future costs. You also need to speak to an attorney who does "Conservatorships" in CA we make a distinction between conservatorship (someone over 18) and guardianship (someone under 18) other states don't make this distinction. Don't be surprised if this becomes expensive. If your mom still has capacity you may not need a conservatorship and a good attorney will be able to help. There are other ways to protect your mom from an abusive sister but a lot depends on your mom's capacity.
Every situation is different, it is important to discuss your legal issue with a knowledgeable attorney in your jurisdiction. To schedule an appointment with me please contact me at 800 220-4205 or www.MartyBurbankLaw.com