Well, your brother violated all four rules that I can see.
First he violated rule one, which is treat the weapon like it is always unloaded. There is no UNLOADED with firearms, the ONLY time a firearm is safe from being fired is if its disassembled to a condition where the firing pin cannot strike a primer.
Second, he violated rule 2, he didn't keep his finger off the trigger until ready to fire. There was no reason to pull the trigger unless firing a gun, there never is. I've gotten into near fist fights with competition shooters, who believe "safe" is dropping the mag, racking the slide, and pulling the trigger. Why the heck you would pull the trigger? This is one of the reasons I dislike Glock firearms and the S&W Sigmas and ANY weapon that requires you to pull the trigger to disassemble the gun. There was no reason to pull that trigger.
Third, he did not point the weapon at a target he was willing to destroy. Thankfully, it was only a minor inconvenience to destroy his window, but it wasn't a target he wanted to destroy.
Fourth, he did not know his target and what was beyond it. Luckily no one was injured, but it could've happened.
I'd suggest something like the Safe Containment ballistic containment pad. Always a safe place to point a weapon if you MUST pull the trigger (like for instance to disassemble said Glock).
Remember, the four rules are redundant. If you remember rule ONE, you should remember rule TWO, which will prevent any extraneous perforation and violation of rules three and four.
-Rob