RE: "Something moved...and he fired."
Tragic, tragic, tragic...and the waters just seem to become muddier. There's no simple solutions here...I wish I had some magic words.
A couple of things though, to the hunter who acknowledged that they sometimes 'cough' to alert a hunting partner who isn't aware of their presence...NEVER DO THAT! Studies have shown that movements (waves), audible noises, throat clearings, whistles, coughs, and the like have led to numerous 'mistaken for game' incidents. Only the human voice should be uttered/shouted to alert nearby hunters.
Next, perhaps fear of the dark, becoming startled, or the fear of the unknown led to the lad's fatal and instantaneous decision to shoot...sadly.
As far as recommendation to the family (baty15jessica), this is all so very sad...my heart goes out to you. It remains an accident...tragic as it is. We all wish we could change it, but alas...we cannot. Unfortunately, your father doesn't seem to be capable of helping your family much throughout this ordeal, and it is a life altering ordeal.
Much like a previous poster alluded to, I too was hunting on my own from age nine, my dad grilled me on tripping/falling with the muzzle of my toy gun (plastic at age 4-5) to always be pointed in a safe direction while hunting with him, placing that toy gun safely in/out of the vehicle was also fundamental. Years later, I wasn't permitted to carry real firearms (loaded) until he had 'set me up' in my spot to stationary hunt. He put me through the ringer and slapped the crap out of me more than once. Firearms safety really became evident while upland bird hunting with birds frequently busting out between hunters/dogs...it's not an overnight lesson but rather ingrained. The worst aspect of the sport we love is that there is rarely if ever a second chance or a 'do over'...I wish there were.
If it's any consolation, I can only hope that your dad spent a great deal of time teaching your brother firearm safety, age isn't everything...as I happen to know more unsafe adults than I do young people. Let your brother know we understand and support him, for there's nobody who will ever feel worse than him. Encourage him to pay close attention to his counselors and in time...learn to forgive himself and to become a champion.