Can someone get an elderly person to sign a POA if they had dementia? My mother had strokes and the doctor said that she could not sign important paperwork.

My mother had multiple strokes at one time years ago and we as a family was informed that she could not sign any legal paperwork because she had problems remembering things and had problems on her left side. Our family thought that our mom had an POA filed out for my sister but she did not so we had to have an health care proxy form done so we could take care of her medical. The family agreed to put me and my sister down just in case I would not be able too. I was able an took care of our mom to the day of her death. My mom had been diagnosed with cancer in 2021. For years my mom has been very ill. We truly needed a POA to get the care that she needed when our mom was giving us a hard time. But since we didn't have one it was nothing that we could do about that. Our mom also was diagnosed with dementia. One of our family members had our mom to signed or forged a POA knowing that our mom wasn't suppose to have signed anything because of her health. We have found out that some of our mother paperwork has been changed and we know that she could not have done it. What can we do about this family member taking advantage of a disabled person with dementia an previous strokes and change paperwork for their own personal gain. The family member had others to help them achieve this thing with taking advantage of the elderly.