How can we control who has power of attorney?

I had durable Power of Attorney and my daughter has second. No one wanted to take care of her because she has no money. Her brother died and his money is being split 5 ways. So now my sister takes our mother to her lawyers and had her sign a revocation of Power of Attorney on me. My sister wants the money our mother is getting. My mother said that she did not know what she was signing. She stays very confused on everything. Is that legal? What can we do to get this revirsed? Is my daughter still power of attorney? What steps do we take to get this solved? How do we protect my mother?