You must tell her asap! Imagine how your family would feel if you didn't and your sister found out you knew about it all along! You must give your sister the chance to help her son get his life back, away from the drugs. If you don't and something terrible happens to him, just how do you think your family will feel about you preventing them from helping him by keeping quiet??? I'm somewhat amazed you even have to ask if you should tell her! This is your sister's child you are talking about here .... even if he is already an adult! And what about the baby??? Are you really prepared to just stand by and allow the baby to remain in a dangerous situation, just because you don't have the courage to speak up? What if this baby accidentally gets a hold of the drugs and ends up in hospital fighting for its life??? Are you really willing to risk that too? Please, act immediately! This is a very serious situation, so take courage and do the right thing, PLEASE!!!