Advice on a Sociopath Sister

Hey everybody,
   I came on this website because I need some help for myself and my parents. My sister is 18 years old and she has been lying, manipulating and stealing  since she was little. I have watched her spiral out of control since last summer and I really want to help her.
   My sister is the daughter of a drug addict. Her biological mom would take her on car drives as a young child to the location of her drug dealer and she would sit in the back of the car as her mom did drugs. My dad (her biological dad) was able to get full custody of her when she was 3 and was able to get her away from her mom.
    When she was little, she stole a girls braclet on the bus. When the principal asked her what happened, she stonewalled the whole time and lied to his face about what happened. He told my mom (technically stepmom) that she was lying just as good as the seniors he sees in the high school. She also had a tendency of hitting my younger brother and giving him deadlegs
   There were dozens of other instances like this, if not hundreds, but last summer was when it got worse. She stole $200 dollars from her grandparents and stole money from her place of employment. She lied about going to work and about how late she had to work. No matter what my parents did she never felt bad about doing it. She also stole around $120 from my brother and I.
   By march of this year I thought she was done with all the manipulation. We left her home when we went on a day trip to the mountains. When we came back my dad noticed that she had put 150 miles on the car. He confronted her and they got in a screaming match. My parents went out to dinner that same day and left me and my siblings home alone. I was in the living room watching tv and I failed to notice that she slipped out the door and ran away. I sometimes blame myself for not noticing.
   Since leaving, she has gotten addicted to drugs and has stolen jewelry from her friends mom (although I thinks she had been doing drugs for longer). She also has gotten in contact with her biological mom and they are both probably doing drugs together.
   Since she is 18 she is an "adult" and my parents can't force her to get better. She only contacts my parents for money and didn't even list my mom as her real mom on a medical form even though she raised her for 15 years. She has also tried to scare my mom by telling her that she was going to attempt suicide. I need some advice so I can help my parents out. They don't try to reach out and all the tension she causes is unnecessary. Is there nothing legal my parents can do to force her to get help? These aren't things a normal 15 year old has to worry about but I need her to get better so my parents can too.

  

[RockRose]

This is so hard,  and this will be with you forever.  Being the "good" daughter and having to watch another sibling wreck the family.  

Enjoy your life,  and your parents,  and don't feel like you have to walk around always being the "good one" - be yourself.  Do what you can to fully enjoy your life and make the most of your successes.  It's possible your sister will come around in her late 20's and do better,  it's also as possible she never will.  

You can't blame yourself for what your older half sibling does with her life.  Nor can you change her.