How to get my brother to stop grinding his teeth on purpose?

Explain tooth damage: Explain to your brother that many teeth are broken, split, need root canals, crowns or extractions as the result of tooth grinding. Does he want the pain & expense that can result from deliberating damaging his teeth?

Ignore it: If you are 36 and your brother is close to that age or much younger and is intentionally grinding his teeth to make noise and bother you...Then i suggest you ignore him. Also, if he persists he'll wear them down and you'll have the last laugh when he needs extensive and expensive dental work.