That's a tough one because I think H's demonstrated attitude has been that he thinks the drugs and bad behavior are your problem to deal with and not his. He'll report it to you, but he won't deal with it. That would suggest you should just have BIL talk to S15 without discussing with H.
On the other hand, BIL is H's family, and having S15 talk to BIL without informing H is crossing the line from a family perspective.
I would probably notify H that you're going to have BIL talk to S15 about drugs. I say "notify" rather than "discuss", because that's probably all H is looking for. If he wants to object to that, you can decide how to handle it then, but for now I would just assume he won't and plan accordingly.
Accuray
Married 18, Together 20, Now Divorced M: 48, W: 50, D: 18, S: 16, D: 12 Bomb Dropped (EA, D): 7/13/11 Start Reconcile: 8/15/11 Bomb Dropped (EA, D): 5/1/2014 (Divorced) In a New Relationship: 3/2015