Angela. Got it.
You have already given her the "speech". Make sure she understands that you care about her, and love her. She is 18 though, and legally responsible for her decisions.
However if she continues in a matter, that you consider to be self destructive, I think you have no choice but to inform your sister. If for no other reason than you are free from blame, if your sister comes back with "Why didn't you tell me?"