You can bring an action in chancery court to have her declared incompetent and have someone (you or the brother who is also concerned about her; not the other two brothers) appointed her legal guardian and to also have the POA revoked by the court. Then as her guardian, you can look into what was done by the brother with the POA and if necessary take action to recover any money improperly taken or used by him.This sort of action is not a simple one--it's much less common and more procedurally complex than, say, a suit in small claims court. You are strongly advised to retain an elder law attorney to help you.You will also need medical evidence of her incompetence. The attorney can guide you in the type of evidence to get.