Hi,
I would also call the police and let whatever happens to her, just happen. Everytime she does something and there is no accountablility on her part, she is getting sicker and deeper into her addiction. There is NOTHING that you or your parents can do to get her to stop using. Until she decides she doesn't want to live this way, she will. She is an adult now, not a kid. You cannot control anything she does, and neither can your parents. Trying to arrange the outcome will make all of you crazy.
I was married to an addict for 25 years and I jumped through hoops trying to make him see that he had a problem. He never did, and I ended up as sick as he is. I lost my own sense of who I was, until I met some people that knew what I was going through, and I came here. I seperated from him almost a year ago, and am in the process of divorce. I am learning how to be me again.
What helped me the most in the beginning was the conviction that unless my stbxh started experiencing the consequences of his actions-instead of me 'helping' him-he would never stop. Jail may be the thing that gives your sister a chance to save herself, and she really is the only one who can.
Love is not enough. Threats of consequences are not enough.
The only way to help an addict, is to stop helping the addict. It just is.