Can my sister keep me from seeing my mom?

My sister has power of attorney over my mom. She is mentally ill and a recovering drug addict who abuses prescription meds. She is violent and abusive to mom. My sister, who also can't see mom, called protective services but my mom is brainwashed and scared said she wanted to stay with my sister. My mom has told my husband and me all the terrible things my sister has done to her but she won't stand up and tell anyone else. My mom was my sister's power of attorney for years and years because of her mental status. Some not so liked attorney still gave my sister power of attorney without any family knowing it. My sister is on welfare since 18 and is now 55. She has been in jail and in hospital a lot. She has cheated the system for years. She now takes my mom's pain meds. I have seen her. Her doctor, who is my doctor also, told me she would call protective services. I know my mom will deny any wrong doing and want to stay with my sister. My sister uses every dime of mom's money which is more than she gets. I don't want to put my doctor in that position either.