My Mom lives with my sister and she does not allow me to see her. What rights do I have?

I am my Mom's POA. She took my mother out of my care some time ago. My mother was with the homecare attendant but that particular day the attendant left my mom unattended to go to grocery store. During that time, my sister removed my mother from the house that we lived at and did not inform me that she was going to do that. My mother was comfortable and there were no abuse issues at all. My sister just decided that she was going to take her to live with her. After finding out that she did infact take her, I did nothing. I let her keep my mother. However, I regret that now because since that time, she has not allowed me to come see her. We have other sister's and a brother that can see my mother and take her to their houses whenever they want, but she does not extend that same priviledge to me. She has no reason for not allowing me to see my mother. She told me that I will see her when she decides that I can. I need help because my mother is elderly and I don't want to see her after she is already dead.