How can I protect my mother who willingly gives money to a granddaughter who lies?

I do have power of attorney. I have tried to explain to my mother that this is not healthy for the granddaughter, who has stolen my mother's credit car number at one point, and ran up $2000 of debits in one month. My brother, her father, is outraged, because my mother is enabling his daughter to support a boyfriend who broke into houses on Christmas eve and stole Christmas presents. The granddaughter has three out of wedlock children from two different men and at one point gave the kids to her ex and now has them back. She posts pictures and posts on the internet showing her lifestyle which includes partying, skateboard equipment, alcohol, etc. my mother is paying for, although my mother believes this money she gives her is for the children, or medicine, or food. The girl has every kind of social service support available. If my brother or I say anything about this, she becomes angry at us, that we are mean, and have no compassion for poor Mary. Recently, Mary had to make a request in front of me because I was staying with mother, Mary said she needed $5000 for living expenses while she began her studies (she's never stayed with a job more than a couple of weeks), my mother wanted to write the check, and I stalled her. I learned from my brother that Mary wanted to money to pay boyfriend's court costs to keep him from going back to jail. We prevailed but my mother was furious that we were controlling her and interfering. I learned that she just gave her $2000 the week before because she put me on her account (Idon't think she knows I can see this). She didn't want us to know. She has told me previously that Mary was "killing her". Because Mary always came with a sad and hopeless story and after she left my mother was so distraught she would take a pill and go to bed, sometimes for a whole day. My Mother is still sharp mentally, but is obsessed with Mary and irrational on this subject. She prefers to think of herself as a virtuous person who never gets angry, so she tells me what Mary, or others, have done, and then let's me get angry on her behalf, and then blames me for being "bad." Help! The entire family is in turmoil and bereft.