Personally, I side with Gauntlet on this. My hard and fast rule is, always, without fail drop the mag and check the chamber(s). I can be at a gun shop, and want to look at something. I will watch the clerk clear and safe the weapon, and as soon as it's in my hands, I do the same thing. Drop mag, clear chamber, safety on. I have no idea if that clerk actually knows what he's doing, and as the gun's in my hands, that makes me the responsible one.
I'm sorry for your brother's error, but the physics of weapons are quite unforgiving of mistakes.
Roger
Yes, once the shot is fired it is too late. He is as careful and knowledgeable about firearms as anyone, but he was forgetful one time ....but one time too many.