Drug-addicted sister using again, please help?

Read my question from a couple weeks ago first: http://answers.yahoo.com/question/index;_ylt=AuAnTmyYK4RxEZHow9cNYxDsy6IX;_ylv=3?qid=20090512184257AAr4hSM Anyways, from reading her text messages, I found that she's also using Aderall for appetite suppression (she has an eating disorder. She's five foot... show more Read my question from a couple weeks ago first:
http://answers.yahoo.com/question/index;_ylt=AuAnTmyYK4RxEZHow9cNYxDsy6IX;_ylv=3?qid=20090512184257AAr4hSM

Anyways, from reading her text messages, I found that she's also using Aderall for appetite suppression (she has an eating disorder. She's five foot three and 95 pounds.). Not prescribed, obviously. She might also be doing shrooms. She smokes weed with kids MY AGE (we're four years apart) during our lunch time and tells my parents she's going to hang out with this clean person while actually hanging out with those drug users.

She's also manipulative and trashy and a total slob. Today, after tagging along with me and my mom to go to the mall and buying forty dollars worth of clothes, she did the bait-and-switch by giving me half of her cookie then having some of my yogurt parfait then saying she wants her cookie back. I get mad at her often for eating all of the ice cream straight out of the jar/stealing my clothes and returning them smelling like Newport Menthols/deleting my shows from the DVR/et cetera. My mom thinks that I'm just being a bully, as if my sister hasn't been a bully to me for fourteen years. They know she's difficult, but would never think that she's using again.

She takes advantage of my parents a lot too. This weekend, they have to jump through hoops so she can go to my cousin's wedding and then leave before the reception because people will be drinking. I sometimes casually ask whether they ever look at her text messages. They say no. I wonder if I should straight up tell them to look at her phone or simply shove the phone in front of them, open to her messages file.
Update: yes, she's been using since she was fourteen but still acts she's four. no therapy seems to help at all. she's un-changeable.
Update 2: GodessVoid and others: Please read the previous post. I think heroin counts as an addiction!
Update 3: oh yeah, and if my parents see the messages, i'd pretty much screw up her life because they won't pay for her college. so she'd probably end up a slain crack whore found on the east side of detroit on the six o'clock news by next october.
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