Wooried daughter

hello. I am 15 years old and my family (me, my brother and mum) is going through a rough patch. Anyway I won't go into that. I know that my mum has been on many drugs in the past and still smokes weed now. However today she pulled out a small bag of cocaine that she forgot about. It's about 2:30 in the morning and I don't know what to do. I asked her what she is doing with it and she said tomorrow she will get rid of it. I made her promise and she did. However, i made her pinky promise and she couldn't bring herself to do it. She has been showing signs of being high but that may be depressing I don't now but im going to school tomorrow and i'm that she is going to take it. Eventually she comploed and pinky promised she wasn't going to use it just after i moaned at her for at least an hour. What do I do and also is everything definitely confidential on here? Thanks..