If he has been legally determined to be mentally incompetent, then you can void the contract for "lack of capacity" (incompetent people cannot enter into binding or enforceable contracts)--though if the parties to the loan (e.g. the car dealership or lender; her, too, if she's also on the loan) don't voluntarily agree to voiding it, you'll need to use your authority under the POA to file a legal action in chancery court (a division or part of country court) seeking a "declaratory judgment", or court determination, that the agreement is void.If he has not yet been officially, legally determined to be mentally incompetent, your first step is to bring an action in family court to have him declared incompetent and get yourself appointed guardian. You will need medical/psychiatric evidence or testimony--e.g., you'll need a psychiatrist who has examined your brother to testify in court--that he is incompentent.You can also sue the woman for the costs (e.g. any car payments made; legal costs) incurred by your brother due to her actions. You'd sue for fraud and/or theft by deception (stealing by tricking someone). Given the number of issues, you should ideally retain a family law attorney to help you. If you have POA over your brother's finances, you can use his money for the legal fees.