My mother made me the executor of her will and POA in 2011. But my sister and niece are trying to steal from her. What can I do?

My mother made me the executor of her will and POA in 2011. At 89 her dementia has worsened. My niece, her husband, and my sister took advantage of her dementia and coerced her to add my niece to the POA. Then they went to all of her banks to get statements. Their intention is to steal from her. I need immediate help.