There is at least one thing that's identical to both cases. The same Assistant District Attorney made the decision to prosecute one case but not the other.
One of the basic tenets of gun ownership, including with BB guns, has always been that you are never to point a gun at anyone unless you actually intend to shoot them. The other important basic rule has always been that you must always assume that a gun is loaded. Most BB guns sold for kids come with that information in the instruction manuals. The parents of the boy are either very negligent for not teaching that or very ignorant for letting their children point a loaded weapon at each other.
In the world of guns, the act of pointing a gun at another person is intent. Most BB guns include manuals that basically have the information stated here. BB guns are not toys and require adult supervision. There are now BB and pellet guns that are powerful enough to be used to hunt small game. These aren't sold to children, and most places that sell them state that they are not toys and are dangerous weapons meant for hunting. You'd have to be a very good shot, but even the low powered Daisy Red Ryder BB gun, which are sold for teaching children how to respect and use guns, can kill a small rodent.
With the above information in mind, why wasn't the boy charged for murdering his brother? Why weren't the mom and boyfriend charged for negligence or child endangerment? Someone actually died in one case, but not the other.