Schizophrenic sister says she has power of attorney over her sister

Can a person who is Schizophrenic have power attorney over her sister? The Schizophrenic said, she can sue the doctor who prescribed oxycodin for sister and did not tell her because she is poa over her is this true? I think no for the both questions.

I think it depends on the situation. I had POA for my Dad, as he was dying, and I’m schizophrenic. As for if she can sue… I don’t think so but I’m not a lawyer.