Any advice on how to go about removing my sister as POA for my Mom?

Mom is 88 with dementia. She prepared a shared Power of Attorney naming sister (who is an RN) and me (I live with mom). She has neglected mom, but won't voluntarily step down. Sister had it changed under dubious circumstances and gave herself sole power. She did so to control mom's finances and has no interest in any of mom's care. Of course, it didn't take long for mom to start suffering the consequences and she is mad as hell about it, but can't do anything to change it herself, due to her dementia. Now mom isn't getting the medical care she needs, and sister delegates everything to her husband, who mom never liked. He gets her upset just about every time he comes over, making the job of actually caring for mom (which I do in her home) impossible. My siblings won't help, as they fear the sister, who is unstable and has history of violence with all of us, including mom. Mom is deaf because sister hit her in the head with a dog food can 15 years ago, so that should give you an idea of how much sister cares about mom. We can't get anything done. We have part time home health coming in to help me, but it's just not enough. How do I get sister removed and the POA passed to my brother, the next in the line of succession? Mom's lawyer won't help, because he fears we're getting ready to file malpractice against him for allowing mom to sign at a time when he should have known that she was not competent to understand or sign anything. Is probate court the only route?