I would be the crap out of him. It doesnt matter what your wife says.
He put his hands on her and he is a grown adult. He is a coward that hits women, you should show him what happens when he does that. As a woman Id expect it from my husband.
I dont give a s***e if my wifes brothers give her shit in any way I will destroy them, if they hit her!!! They wouldnt know what hit them. Be a man, its your wife, if someone on the street hit her what would you do? I would hope you would go out and belt the b*****d!!! Its no different family or not!