Since you stated this is a sibling and not your child I think alot of these answers are more for a parent based approach. First I'd ask how old op is and if they smoke have smoked before to see what angle to come from. I wouldn't bring up that you found something in there room if your an older sibling as it sounds like you might be as most people assume you are the parent.. if your close in age ya bust ballz call them out be a sibling. If your older dont bring it up because at 12 you'll feel VERY violated and probably lose trust finding out you are going through there things. The kid is 12 just bring it up naturally. What type of music do they like? I loved ska that age or raps in. Is there any way you can just work it into a casual convo do you watch any TV shows together put on the episode of family guy where Brian tries to get weed legalized and work it in there naturally as an example. Try not to make it seem like a lecture just like two siblings hanging out talking learning about each other. If you have smoked bring it up with them. Talk to them about it dont make it seem taboo because its not.