How do you get power of attorney of your parent?

My uncle lives in the house with my grandmother he has not been taking care of her but he will not let my mother take her because she gets a check. My grandmother was admitted into the hospital at the end of September her leg was swollen and she was malnutrition and she contracted a bad infection she stayed in the hospital for three weeks. My cousin found her in her room were my uncle locks her up she had bowel on her and she was in pain. She is now in a nursing home after being in a hospital my mom has been with her every since . My uncle wants to obtain custody of her after her rehab is over what can we do to stop him my grandma cannot go back to those conditions he is a convicted felon he only wants my grandma for her check can you please tell us what to do that want cost us a lot of money to get custody of her some way were she want half to go back to him please help us please.