Welcome to BPDFamily, happy to meet you, sorry you're here...
Just a quick FYI, she's likely not "abusing" Xanax in the traditional sense if she's BPD - it's more likely actually a good thing for her to be taking even if proper titration is not her strength. It can be addictive and it's possible to overdose, but even if she's taking 3mg/day then it's definitely not a problem (that's 12 tablets of the "normal" size, or 3 super-strength tablets), even doubling that number isn't likely to cause her serious harm. Remember that it's always a balancing game, sure there's an minor risk of liver damage from overuse but realistically it sounds like your step-daughter is at far more risk from her own erratic risk-taking behavior than from anything below the neck.
The Adderall might be necessary if she's got symptoms of ADD/ADHD, but it's also often used as a substitute by BPDs instead of something more effective like Xanax if they can only get their hands on Adderall, etc.
The cocaine and alcohol are definitely real problems. In my very limited experience, cocaine is an easier habit for the mentally ill to break than alcohol is. Alcohol makes her feel soothed, numb AND "cool" AND parasuicidal all at once. The cocaine is more likely to make her BPD (or ADD/ADHD) worse not better, even in the short-term so it's less likely that she even "enjoys" it as much as the alcohol.
Were I in your shoes, I would focus the intervention just on the cocaine - it also allows you to scare-monger that you've heard all these stories around her town recently about cocaine arrests, long prison sentences, etc. Can't do that with the others ;)
