My oldest niece has obtained POA over my mother without consulting me. Did she have the right to do this?

I've been casted out by my sister (5 yrs older than me) and her children and their children for many years. She's been taken care of by our mother all of her life and still is at 58yrs old. I recently found out about one of our mothers credit cards having a past due amount and my sister is to blame. Our mother is 88yrs old and starting to lose her memory. I confronted my sister and threatened her and her boyfriend of being guilty of elderly abuse. Now her daughter, my niece has filed P.O.A. on our mother primarily to keep me out of the picture. What can I do?