Can a person be prosecuted for stealing if they have been a legal signer on a bank account for 10 yrs?

My brother-in-law has has signature authority on his father's bank account for 10 yrs. His father fell ill about a year ago and made him power of attorney, as well. In the last 6 months, the brother-in-law has taken over $36,000 out of the account for personal use. We know this because the ATMs used were mostly at strip clubs. His father recently ped away. Now the other brother and sister are threatening to throw in in jail. Can they really prosecute their brother? My brother-in-law has said that he will pay all of the money back from the inheirtance.