How do I go about becoming POA for my Mom? My sister is currently POA.

Okay, My mother was diagnosed in Feb 2013, with Dementia/Psycosis, my sister became her POA in May 2012. I moved back home to help my mother in June 2012. My mother's house was rented out in April of last year so me and my mother got a smaller house, my mother pays 800.00 for rent. I pay all of the utilities and food and and provide my mother with mani/pedicures I work everyday, I pick up and pay the small co pay for her medication, my sister lives 2 blocks away and does absolutely nothing,my mother's house has now gone into foreclosure, (have proof) my mother's tenants was paying rent my sister was not paying the mortgage company with that money, I have NO ACCESS to my mother's money, I know NO ACCOUNTS #'s NOTHING everything that I do for my mother comes out of my paycheck, my sister does not even volunteer to do anything for my mother but spend her money, I have 2 brothers and they have no idea whats going as well, but I do, my mother is a retired R.N. has built a wonderful life for herself it sickens me that everything that she has worked for she is losing, I'm my mother's primary caregiver, my cousin has moved in with us to help me unload the pressure off of me, I have no children I'm single and have given up my whole entire life and moved back home care for her and, if I had to do it all over again I will not change anything. Please advise me in how do I go about doing this before my mother have nothing left, my mother is 69 years old and has built homes and traveld and retired nicely, and they way it's looking she won't have anything my sister is taking her money and spending it on her own lifestyle