My father doesn't want to be around my brother. What should I do as the POA?

My father appointed me POA for a big reason as he trusts me and not my other brother. My dad is the one who said lets get a lawyer. He was also diagnosed with dementia and my sister stole a large amount of money when I was away for one day. My father doesn't want to be around her our my brother. What should I do? The poa was from a lawyer and his dementia test also was done the bank said im in full control of his finances and health and his estate