How can we find out who is POA and can we evict the non-POA from a home?

My grandparents are in an adult home. My father believes that he at one time signed as the POA of both of them.

Now my aunt had gotten kicked out of her apartment because her landlord felt she was unstable and mentally ill. She then went to the doctors and asked for a note to be the caregiver of my grandparents so she can stay in the house.

Since then she has limited the time my father and mother can see them, and as per my parents they (my grandparents) are looking a little skinny and have been not the same since my aunt moved in. They used to talk all the time and see them but now they tell my parents that they don't want them their anymore because my aunt said that they sneak around and go into their rooms.

My aunt has also called my sister's college and accused her of recording a visit she had with my grandparents and tried to get her kicked out, along with accusing my brother (who is in medical school) of stealing things from the house. My aunt has taken control of the house and the financial part of my grandparents' lives and has convinced them that my father and mother are bad and not to talk to or see them anymore.

Also when they did go over they were not allowed to see them alone without my aunt there. My question is how can my dad find out if he is still the POA? Because he lost the paperwork. And how can we get my aunt out of the house being as we feel that my grandparents may be in harm, mentally and possibly physically? And what rights does she have as a caregiver?