Your brother is just wrong to let his dog bark all the time. He should get control of his dog, however that needs to happen, or give the dog away. It's incredibly selfish to let this go on.
He has no right to make the neighborhood a miserable place to be. I doubt that getting someone wet with a hose is 'assault' in any state, but I have to say I would have been tempted too.
I also doubt that the neighbor with the hose is the only neighbor bothered by this.
On the other hand, the neighbor could have come over or called up and asked your brother to take the dog in. Unless your brother has already stonewalled him.
At the end of the day, both are wrong and need to try harder to be civil. They'll probably have to be neighbors for a long time. This stuff can escalate and get very dangerous.