<human>: Can you summarize the given text in a few sentences?

A bit of background information: I've been a lurker now for a few months now, but a recent family issue has drawn me out of hiding. 

My younger sister (2 years younger) recently had a friend over while I was downstairs studying/on reddit. Keeping in mind that our house isn't very sizeable, I was able to hear their conversation with fairly decent accuracy. What I was able to discern from their talking, was that they had both consumed some sort of drug (unsure of which kind) though, I am certain that its not marijuana (I have been down here the whole time but would have smelt/heard the process of marijuana consumption) Snippets of what I overheard are: "how will it take to kick in", have you tried something like this before", "is your brother cool with drugs". Shortly before I came downstairs to do my homework, my sister had asked me for some money (15$) to add to what looked like about 20 or so dollars in her hand (I'm hoping that the amount of money can provide some indication of what type of drug she bought). After acknowledging that I had now money to lend her, she promptly left the house for a brief period of time (presumably to make a transaction with a dealer), leaving her friend alone in her room. I myself am a recreational user of weed, and have suspected that she has has been too throughout her high school career but nothing of this level. My sister has had a history of hurting herself over relationship troubles (one incident), problems in school, to the point of family therapy, yet I still consider her to be a very sweet girl. I know this is alot to read, but I feel that I've never assumed the role "protective older brother", yet this would be a very crucial time to step in as I am very wary of drugs other than marijuana or alcohol. Any advice on how to approach this situation would be very much appreciated. For now I will simply be keeping an ear on her.
<bot>: My sister is experimenting with drugs (not marijuana) and I am unsure of the appropriate way to deal with this situation.