What you need to do, in my opinion, is find out everything you can about these herbs.  Ask your family doctor and also, you can look them up online if you wish to.  If you have seen this activity, of even if you just suspect it, and know the parents or guardians of your nieces friends, perhaps you need to discuss this problem with them.  If you have information, and can talk to the other adults about it, together you can devise a plan that will be most effective in explaining the long term affects of these herbs on the body and mind of these children.   If you don’t know how to approach this subject, and how to set boundaries for behavior, of this child, in your home; it might be best to speak to a family counselor.  They are trained to teach you how to speak to your niece with respect, and yet, authority.  This way you show respect for her feelings and ignorance of her behavior; and still you demonstrate that her actions are harmful to her, and unacceptable behavior for both of you.  GET INFORMATION, and get help.  Don’t try to handle this on your own.  It is wonderful that you are reaching out for solutions to this problem, now, before it escalates into something worse.