Can a POA be overturned if the POA shows no commitment to a loved one's care and is clearly only money driven?

Hi all, this is my first post so be gentle! I am heartbroken. My brother has controlled my mother for many years having lived permanently with her until his early 40's. She fell into a deep depression when my father died 6 years ago. Unbeknown to the rest of the family he obtained POA when she was compos mentis. He stopped her answering the phone and we had no responses to letters etc, saying 'mum wants to be left alone' but now we have learned that 15 months ago he placed her in a rest home 150 miles away from us and sold her home. She now has the early stages of Dementia. The home is unaware that she has a daughter, granddaughter and elder brother and other extended family who would visit, in fact they are horrified. Since putting her in the home he has not visited her ONCE. My question is, forget the financial side of things, are we able to transfer her to a home closer to us so we can visit regularly or can his lack of commitment to be enough to overturn the POA? I just don't know where to start with all this. Thank you