Yours is a tricky question because POA may or may not be in place due to lack of mental capacity. However, the dealer does not know that. Tell the dealer that your brohter does not have the capacity to enter into a financial agreement so it would be in their best interests to take the car back.
Then consider filing a petition for guardianship.
The question is whether you brother is: 1) under the age of 18 and/or 2) declared incompetent by a guardianship court. If the answer "no" then he is presumed to be competent. In any event, it might be wise to consult a local attorney that is familiar with consumer transactions.
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