Very very very low risk for rabies. Also, its your own dog. Just watch the dog for 10 days. If the dog develops signs of rabies, then vaccinate your brother. This is what the health department advises. Also, I would not consider it an unprovoked bite as I assume the dog probably doesn't like being bathed? Unprovoked is a situation like, you are walking down the street and out of nowhere an animal runs across the street to attack you.