Dani,
First of all, my heart goes out to you because I know how painful it can be when a person you love lives a destructive life.
My situation is a bit different than yours but maybe my story can help you "stick to your guns."
My 20-year old son is an addict. He, too, struggled after his father left the family when he was only 7. It wasn't the divorce that hurt my son, it was the fact that his father lost interest in him afterward.
I remarried a man that took my son under his wing and was an incredible stepfather to him but nonetheless, it didn't fill the void. My son started down a drug path junior year of high school, dropping out as your sister did, just a couple of months before graduation.
I had him in therapy when he was younger and all sorts of evaluations as he got older when it was obvious that he was really struggling.
What started with marijuana had become a full-blown heroine addiction by age 18. I spent the next year of his life, continuing to help and seek out the best care, etc. He had off and on jobs and I paid his car insurance when he was out of work, doctor bills, etc.
I did everything I could to keep him safe, even after he started stealing from us. I just thought I had to work harder to help him and I did everything humanly possible.
I didn't know it at the time - I had become his enabler.
I am telling you all of this because, until I stopped helping him, his behavior never changed. I have small children in my home and I could no longer help him at their expense. I evicted him from my home and gave him nothing but a meal here and there.
Sounds harsh, yes. But a recovering addict told me that I was not helping my son. By continuing to enable him I was "keeping him from feeling the pain he needed to feel to get help." Boy - that hit me like a ton of bricks.
So Dani, as hard as it was to say no, and believe me - I know that was hard on you. You did the right thing. Yes, she may end up homeless like my son did and she may get angry at you and cry to you and do whatever she needs to break you down to give her that help. Stay strong. It is what is best for her.