My 15 year old Sister is using drugs, what should I do?

My 15 year old sister and I live in different countries so we don’t spend a lot of time together. I know she has been going to a lot of raves recently, and posts pictures on FB hinting very well she was going to “roll” and take MDMA/Molly. So my girlfriend confronted her about the pictures and confessed that she tried it for her first time. This was three weeks ago. During this time I have been so confused on what to do. Should I even make a big deal about? Tell my siblings that do live in the same city as her? Tell my parents? Do I want t freak them out over one time?
Well, this passed weekend she told me that she went to a two day rave and took MDMA again. So this time I did panic and tell my siblings. We are all worried, yet we still don’t know how to approach the situation. I’m afraid to confront her about how feel, that she might close up and shut me out, and stop sharing about her life. And start lying to me like she does to the whole family. I know I can’t just do nothing because if something were to happen I would regret it for the rest of my life. I know its not an addiction, but could be the beginning of a very slippery slope. Any ideas? Thanks!