Well, Im very young only 18 and I never smoked pot but I can honestly say every single one of my friends did, and I mean it's not horrible, I think you should discuss it with him and amke sure he knows the effects of it and what it could cause but yelling at him and grounding him is going to do nothing except want to make him do it more. I remember back when Iwas from like 13-17 anything that my parents grounded me for I did it again except worse just to piss them off. If your son is gonna somke pot, he's gonna smoke pot weather you yell at him or not, so I think the best approach would be to amke sure he knows what it is and what it can do and tell him he shouldnt do it and explain to him why, and even make up a bad story about it and one of your highschool friends that had a bad ending to scare him out of doing it or something.
But I think I can speak for most kids and say that grounding and yelling and smacking them is just what makes them want to do it more. And also when parents shelter there kids to much it can cause bad things when they finally turn 18 because they have so many freedoms and don't know how to handle them because when they were 17 they didnt have any so it can cause them to make bad/stupid decisions.aman

I'm extremley sorry for what your going through and I hope that all is well soon.

Your sister is in my prayers, best wishes and good luck with whatever has to go down with her.