As per Indian laws, a Power of Attorney (POA) executed outside India needs to be attested by the Indian consulate in the foreign country or apostilled by the competent authority of the foreign country where the POA was executed. If the POA is notarized, it needs to be authenticated by a notary public or a competent authority in the foreign country.
In your case, since your brother is a US citizen and the POA is being executed outside India, it would be advisable to get the POA attested by the Indian consulate or apostilled in the US. This is because the POA needs to be registered in India, and the authorities in India may require the POA to be attested or apostilled for validation.
Alternatively, your brother can execute a POA in India and sign it in front of a notary public or a competent authority. This POA would be considered a valid legal document in India, and it can be used for the purpose of renting out the property.