My 8-year-old son plays with me as DM (42) and others ranging from a mature 13-year-old, a bunch of 20 somethings, and a 35-year-old. He plays a Dragonborn Fighter, so the dumb brute thing is his character, but we've had to set him straight about not just killing everything. I even designed an encounter around it where he was falsely arrested by a bunch of armed guards, so had he fought back, he would have died on the spot. He did the right thing and went willingly (which then provided some key information pertaining to the adventure), but the goal was to "teach" some humility and thinking before acting. I knew how my son would react, so I could do something like that and get to the desired goal.
Perhaps you could work with the older brother for something similar. Maybe the local thieves' guild takes an interest in the naughty character, kidnaps him, and offers him death, a total stoppage of his criminal activities, or acceptance of a measured, unionized life of crime (giving you the opportunity to provide him outlets for his "crime" without disrupting the story).
Perhaps you could work with the older brother for something similar. Maybe the local thieves' guild takes an interest in the naughty character, kidnaps him, and offers him death, a total stoppage of his criminal activities, or acceptance of a measured, unionized life of crime (giving you the opportunity to provide him outlets for his "crime" without disrupting the story).