As others have posted, your anger, hurt and rage towards this young woman is evident to all of us and I'm sure quite evident to your step-daughter. If I'm reading the original post correctly, you have been a mother figure to her since she was about 7 years old. You've had sole legal custody of her for the past few years. All was well, you became "best friends" (#1 mistake). As posted, even with your own biological kids, you are never, ever to be their best friend. You are always, and at all times, to be their parent (step or not). #2 mistake was letting a young impressionable child go and visit an arsonist, drug using, lunatic mother who had no legal rights. Would you have let her visit anyone else who fit this description? But, I'm guessing as her "best friend" you didn't want to stand firm on the no-visit policy. Please correct that immediately.
Second - as a mother and a step-mother I can guarantee you, had this been your own biological daugther you would still be facing some of these same issues. When their hormones change - they change. The sweet little baby girl that couldn't wait to crawl in your lap to be cuddled to sleep will suddenly make Linda Blair look good. All the more reason to Stand Firm - be the parent, not the friend.
We had a saying in our house, or at least I did with my three children, especially during the years as a single mother and they were each a year and half apart so 3 pre-teens/teens at one time - "This is not a democracy. You don't get a vote."
When DH and I married and he had two kids, same ages as mine and they decided to go live with their mother who was never home and had no rules we fought and lost that battle (they were 16 and 18 at the time). I still remember one trip to NY (we took all 5 teens) and his daughter stole his credit card, snuck out of hotel, went shopping, hid merchandise. When we found out later and he confronted her, she denied it, blamed it on my daughters, and then told a huge lie about how I had taken the three girls (my two and her) shopping and forced her to stay outside Tiffany's while I took my two shopping inside. So, yes, I know where you are coming from - that was the start of our marriage 10 years ago. It's been a bumpy ride ever since. But, I still stand firm and act like their parent and treat them the same as my own - they are all grown now, married and having kids of their own. The mother's influence on his two now grown kids is quite obvious but we still present a united front and peaceful home for them to come to. They are all our kids and we don't use the "step" word. My kids love and respect their "step-father" more than they do their own father (who has lots of issues himself).
You must deal with your feelings and love this child unconditionally. There were times with my own children that I would say, jokingly but not really "one of us is going down and it ain't gonna be me." Stand firm.