Can someone change my mother's POA?

My mother is 96 and is getting confused and forgetful all the time. She has previously designated my eldest sister as her POA and which is a fine decision. However, our other sister is now attempting to change the POA to herself so that she could access mom's money in banks and probably change the title of house. We need to know how to protect our mother not to let my other sister change herself to the POA. The second sister previously misappropriated my mother's money and other possessions. We chose not to go to the police at that time though. Probably to save herself some trouble, my mother now has a tendency to say yes to anything when someone nice to her asks. So we have ground to worry.