How do I stop a nephew from becoming a conservator for my mother?

Question: New to all this and not sure how to start.

How can I stop a nephew from becoming a conservator for my mother who was recently put in a home for dementia? My nephew has tried to take over her finances twice. Underhanded nephew also went through his mother's money before she had even passed spending all the money. He was holding onto my sister's will and not passing it to my father (who was supposed to handle things) as requested in her will. He also got my 83 yo father involved in an unscrupulous deal with a contractor and my father ended up owing the contractor money. My nephew lives in my father's house (most likely rent free). My father doesn't want to evict him because of the grandchildren that live there. Any suggestions on how I can stop my mother from being taken advantage of? Also, how can I protect my father from my nephew? I am out of the loop as my siblings won't talk with me and are ignorant to my nephew's shady dealings. My father told me these things. Thanks.