I think if you pick one of the brothers and go and find all the pictues for that brother, you can relabel them. It will keep the other photos with the first brother's name. It will also keep the brother's name you just added as a new person. After doing this, maybe going forward, when you add new pics, it will start to tell the difference bewteen the two.
Unfortunetly, and to your point, there is no way to do this in a batch.