I'm kinda in the position your little sister is in (with the exception that my brother is 25, and I'm 21)
My brother recently got called the cops on by my parents because he was shooting ketamine in the middle of the night and he kept falling and shit keeping us all awake, and my parents REALLY needed their sleep due to work day after they really need their concentration for or they're ruined. This resulted in drastic measures and my dad just flipped out and called the cops
Anyway, I've been taking drugs on and of (mostly just experimental) for the last 4 years, but my parents never know, as the one time they wouldve found out, my brother took the blame (saying it were his drugs) as my parents knew about him (though they never thought it'd be as bad as IMing ketamine)
I'm now in a really really awkward position because i'm still semi-experimenting (it turned into somewhat of a habit, but i take months and months of breaks, so I wouldnt say i'm addicted... I even take breaks when I have a pretty big stash lying around), but if I get caught now, it'd completely devestate my parents and i'd get shit slung at me like "didnt you see what happened to your brother?!!!!"
but the reason I think my brother was in a worse position than me to get caught by addiction is because he didnt have a job, didnt go to school,... basicly he was just sitting home all day doing nothing. In other words, he had nothing to live for
I on the other hand have a beautiful girlfriend, I still study, and I take my studies really really serious (i'm top of my class), so I definately have something to live for, and I live towards the future whereas my brother didn't think further than 15 minutes from now
I'm not really comfortable in this situation, but I don't want to be fucked over just because my brother made mistakes. Some people CAN keep control over their drug use. I don't know if I'll always be able to control it, but these past 4 years have shown me that I have at least a pretty good system to keep things under control.
(sorry if this seemed a bit off-topic... i just thought i could relate to the situation, but from the other POV)
Edit: it might be worth nothing though that he was the one that "introduced" me to drugs. My first experience with drugs was when I was 17 and I did LSD with my brother and his best friend. That is my very first drug experience, even before Alcohol (I don't like the taste of beer, and a couple months after my LSD experience i got shitfaced on alcohol, decided I didnt like it, so ever since never drank alcohol other than the champagne during family gatherings

)