I think you don't have anything to worry about with the cops. If your Grandmother already knows then you don't have to worry about your family being pissed off. The worst that can happen is that the mom starts talking a lot and you get a bad rep in your community. But I really think you should stop dealing. If she calls the cops they probably can't do anything to you, but it will make them watch you more closely. But if you're like me, you'll always think the worst and worry yourself far more than you should. Hope this whole thing blows over for you.