How to enforce a HCPOA in a memory care facility

My brother had my mother sign a Durable POA, Financial and Medical while she was hospitalized and in poor condition (her doctor said it would be difficult to assess her capacity because she was “in and out of lucidity.” I had been my mother’s HCPOA for two years prior to her illness. Two months after my brother had her sign the Healthcare POA, I asked her if she wanted me in charge of her healthcare. She did. She signed the form and it was witnessed by 2 people. She’s at a new facility and they will not accept it saying they have a question of validity. My brother said it was not her signature. How do we get this resolved when my mother now has a diagnosis of dementia?