So, the younger kid loses at a video game, then goes and gets a shotgun and loads it as a bad joke (OH HI PARENTS). And the older brother takes it away from him, the gun going off in the process.
It sounds like an accidental suicide, due to their father's negligence in securing his firearms.
Oh, wait, I mean:
The brothers were playing a death simulator, and the older bother, fresh off the high of virtually disemboweling his younger brother, gloats maniacally. The younger brother, driven over the edge by the combination of the adrenaline from murder simulating and his recent e-defeat, remembers where his dad stored his shotgun. And every gamer knows shotguns are the best short range solution in a firefight.
He carefully loads the barrels, but his older brother recognizes the familiar sound of a shotgun loading near the area of the weapon spawn point. He charges in to melee range, prepared for the inevitable quicktime struggle. Then, the older sibling valiantly mashes his brother's face to disable the shotgun, but fails to stop short of the final button press that resulted in an instant kill. In the cutscene after the battle, he realizes that his enemy was actually his younger brother, and unlike their other battles, Aeris dies.