Dude, if you're an addict you should realize how silly the notion of seeing someone else having drug problems working as a deterrent is. That's not how it works, if it was then drug use not only wouldn't run in families, but it would be the exact opposite; a child who, say, had a parent who was an addict would never, ever touch a single substance lest it fuck up their life as badly as they watched it do to their loved one. If both you and your mother suffered from addiction chances are pretty damn good that your sister is going to go down that road, and unfortunately no matter how much you do to try to help sometimes it's an unavoidable outcome. Ultimately it's up to her, and again, I'm sure anyone who's ever done the entire addiction thing can say for a fact that when we were young there wasn't a damn thing anyone could've said to change our minds about using once it was set or to convince us that drugs could fuck up our lives.
Unfortunately other than lending support when she needs it and doing whatever you can to help her stay on the straight and narrow there isn't much you can do. There's a reason parents and family members get so frustrated and desperate when their loved ones struggle with addiction; it's a really helpless feeling knowing you can do nothing but sit by and hope that the person decides to change for themselves, as that's the only way it can happen, and it's incredibly difficult being forced to emotionally distance yourself to the point where if they go down you don't get dragged down with them.
Granted, I've never really experienced that side of the equation, only the addict side of it, but that doesn't mean I'm not conscious of how hard I and others like me make it for our families. The way addiction turns a person against themselves and their family and makes them want to take advantage of how much their family wants to help is such an awful and tragic thing...I really wish we lived in a world where intolerance didn't exist, because in such a world something like addiction would never be permitted. No person could know of something that causes so much suffering and turn their back on it. I guess there's no point in imagining such a world though, because in this one, in this miserable, unhappy world, many people would rather sweep such unpleasant issues under the rug while pointing fingers to try to justify writing off their suffering fellow human beings or even actively kicking them while they're down in a pathetic attempt to make themselves feel better about the insecurities they have about their own life.
But regardless, good luck. The issue you're facing is never an easy one.
