What do your siblings think about Marijuana or other drugs?

[Inferius]

ya, but how do i start the conversation..?

That kind of depends on how you two interact.
If you wanted to be mature about it, just wait till the parents are out of earshot, ask him to come into the room, close the door, and say something like,
"I need to ask you something, but you have to promise not to tell the parents" or something like that.