Is legal or right for my sister to ban me from visiting my mother in anursing home, i was just escorded out by police?

First, I am sorry to hear about your situation. I am sure this is frustrating and emotionally draining. Without knowing anything of your family dynamics, I can tell you that the POA likely allows for the agent to exclude visitors. Again, without knowing your family dynamics, I can't imagine it is the right thing to do. You should try to talk with your sister to work out your issues. If that cannot be done, you may have to retain counsel for assistance. Good luck, I hope you work things out.