What Have You Done Little Sister?

This morning I got a phone call from my mom. A very sad and dissapointing call. My little sister, Jeannie (remember, she came to visit me this summer:
Sister,Sister, Medieval Times, Around Campus, Vacation Plan) has moved out. I had a feeling this would happen eventually, but I had really hoped she would at least TRY to be an adult about it. Instead, she came home late from work (around 11) last night, told mom she was home and "went to bed." This morning, when she wasn't in her room, mom went in to find a NOTE left on her bed. It pretty much said that she was tired of being treated like a kid (she's 20) and all she wanted to do was be able to go out with her friends and have fun. She didn't want to have to tell mom & dad where she was going, where she had been, or who she was with. She said that she was "sorry" it "had to be like this," but she didn't know how else to tell them. She also said that she would go to the house today to get her stuff.
Apparently, she was scheduled to get off work between 3 & 4. When she wasn't home by 4:30, dad went to see if she was still there, and her car was parked outside her work. Mom called to talk to her, but nobody answered the phone. It's now nearly 8 (their time), and there has been NO WORD from her.
I am extremely sad and hurt by all of this. Who knows how long she has been lying to all of our faces: "I love you, everything is fine." And EXACTLY what are her reasons for leaving? Is it a GUY? A GIRL? Are mom & dad really that unbearable? (I know they do get a little annoying sometimes, but they really are good people.) Is she even in school (or has all that been lies too)?
See, I have a lot of "feelings" about some things that she's been getting into, and it's not a very good road she's headed down. I know for sure that she drinks & parties. I also know that she "sleeps around" (but to what extent?). I think she may be into drugs, and maybe other "extracurricular activities." But I think she's just one of those people who thinks all of that is okay. (I know lots of people feel that way, but I never have.) She's also gotten piercings and tattoos (when she was underage) then lied to mom & dad about them.
I know mom & dad are mad and really HURT, but I was mostly concerned for my baby sister, Ruthie (14). But, after talking to her this afternoon, she almost sounds "relieved." Jeannie has been using her as a "sounding board" when (I guess) none of her friends are available. So, my 14-year-old sister has been hearing ALL about the WANDERINGS of her 20-year-old big sister!
I just pray that the mistakes Jeannie is making (and is going to make) are not the kind you can't TAKE BACK! Tonight I've tried texting her (her preferred method of communication) and calling, but can't get through. In fact, she told mom & dad that her phone had been cancelled. But, when I called the message said "At the subscriber's request, this phone is not acceping incoming calls." And, there's no option to leave a message.
A little back story: About 3 years ago (about a year after they moved to NM), Jeannie had gotten in a "little" trouble. One of her co-workers (a guy who already had a 1-year-old and another on the way) was getting "involved" with her. The only way mom & dad found out about it was when his girlfriend (or ex) kept calling the house harassing mom & dad & threatening Jeannie! Mom & dad were so mad they told her to GET OUT! But, when they actually saw the guy, they realized that she'd probably end up pregnant, knocking back on their door in no time, so they told him to get the HELL out and "punished" her. I think the punishment included her quitting her job, and being banned from the car.
I know all of this may seem like "normal" teenage rebellion or whatever, but there's more to the story. I think I've talked about it before, but here goes. When I was a sophomore in high school, mom & dad got a call from her mom (my dad--step-dad, technically, is her bio-dad) saying that she "couldn't" take care of "his kids" anymore & they needed to go get them. So, they rented a uhaul and drove 12 hours to Orlando, FL, packed up their stuff and brought them home. That was back in 1997 (wow, it seems like SO MUCH longer ago than just 9 years). Since then, they've been provided with everything any kid could need. Mom & dad even busted tail to get Jeannie CLOSE to caught up in school (she graduated this summer at 19-years-old). She has new glasses, had braces for a year (just got them off last month), and dad even co-signed for her NEW FREAKIN CAR (I got an 11-year-old pickup truck when I graduated)!! She has never been spanked, grounded (besides after her "episode" with that one guy), or deprived of ANYTHING.
So, it's not like she's kept under "lock & key" and she's whipped every night before bed. Heck, she's never even been given a "curfew" (I wasn't allowed out on school nights and I had to be home by 11 on weekends--12 on prom night)! So, where does she get off?
What would you do if this was your daughter? Should she be allowed to take whatever of "her stuff" she wants (i.e. TV, radio, laptop, furniture, etc), or just the stuff she's bought with her money (i.e. clothes, CDs, etc)? Should mom & dad change the locks so that she can't come in the house unless they're there? What if she tries to "sneak in" tonight after their in bed, or tomorrow when they're all at work/school to get "her stuff?" Finally, what about the car? I seriously doubt she will be able to keep up payments for long (she has NO IDEA about the price tag that comes with ADULTHOOD), and it would end up falling on dad. Should he just take the car and give her their other car (the 6-year-old Kia she was going to buy from them before she got the new car)?
Anyways, I just needed to get all this off my chest. It's a new development in my Family Saga, and it will be "interesting" to see how things unfold. Thanks for listening.