hmmm, how would your brother take it? does he smoke? know you do? My first thought would be to talk to your nephew, you say you guys are close so that shouldn't be too hard. However, if my brother knew my kid was toking I would really want him to tell me as well. I'd probably talk to your nephew first to get his side of the story, how much and how long, probably talk to him about the importance of school and all that jazz.