What can we do to protect mom from financial abuse and be able to rest in the rest of her life?

My mom is 86 year old and three years ago, she was no longer able to live by herself. She moved to my sister’s house. The sister complained that she has no enough room to take care of mom. Mom sold her house and gave her the house money ($14,000). But she wants to buy a nice and new house and she wants more money from mom. Although she disagreed from beginning to give the sister her saving, she did it under the pressure. Mom gave her total of $15,000 for purchasing the new house. She is getting the money from IHH from county and half of Mom's pension. After three years, she asked more money from mom and arguing with mom a lot lately. I have a brother and another sister. My brother and I is the person to take her to all the doctor's appointments and take her to the bank or outside for lunch or dinner.