There are no laws concerning storage of firearms inside the home in NYS. Dunno about where they were located though. People are supposed to use common sense at some point in their life.
Chances are that the gun is his brother’s every-day carry gun that he walks around with. I don’t know any that carries that doesn’t have it loaded with a round in the chamber, ready to go in any situation. In this situation, that didn’t work out too good though. Should he have unloaded it at home? Maybe. That’s not for us to decide.
Brother shouldn’t get any charges brought upon him. His brother was 18, and it’s his own fault (or his parents) for not knowing proper gun handling and safety.