You said a loss of all privileges for months to a 9 year old for a decision where the outcome for outweighs the actual act.
Your answer was for retrobution, not to teach a lesson or change behavior. Make the kid help his sister for the next few months. Have him do her chores. Teach him a lesson.
But loss of all privileges for months on end for a 9 year old is nuts. What if he threw a ball at a brother who had a hidden aneurysm and it killed him?. Do you ask the authorities to complete the execution or just handle it at home.