I agree. If he did injure the dog, he may need counseling. This kind of behavior generally only gets worseIf her tooth loss is due to a traumatic injury she needs to see a vet. It sounds like she is in quite a bit of pain if she is having difficulty eat.
I would also advise that your brother not be left alone unsupervised with ANY of the animals. This is very concerning if, in fact, he did cause the dog this injury.