Concerned Dad in LW1...  In your home, you set the rules and your BIL should respect your "no guns allowed" rule.  Outside your home, your BIL is free to do as he pleases and you have no say in the matter.  If it bothers you that much, you're free to keep your kids away from your BIL.

If it's the sight of the scary gun that bothers you, perhaps you and your BIL could compromise and he could carry concealed while around you.  

And just FYI, your children are far more likely to be injured while riding in your car than by your BIL's firearm.