I was a stepdaughter once too - my dad and stepmother got married when I was almost 10. And when I was about the age your stepdaughter is right now (maybe a little younger), our relationship went to hell in a handbasket.
Part of that was me doing stupid teenage stuff - I was a pill for my biological mother too - and some of it was her not realizing what she was doing. It was a hard, hard, slog, and it lasted off and on until after I was an adult. Things weren't completely normal until about three years ago.
We had to work it out - the two of us, independent of my dad, who wasn't really good at standing up to (or for) either one of us. As a teenager, I didn't care if she liked me...but I desperately wanted to know that she loved me. It smarted to see her treat her own kids differently than she treated me. My parents divorced when I was a preschooler, so I had/have little recollection of their ever being happy together - which probably helped, as I didn't feel like she was taking my mom's place.
And as an adult, I finally had to say (after several long, protracted, stupid fallings-out), "You all are welcome in my house, but your bullcrap is not." And after a few go-rounds of this, we have all grown up and learnt how to talk to each other as adults. Things are better, I'd even venture to say they're great, and we talk on a fairly regular basis.
You may not ever be 'good friends' with her, but with a bit of time and space you'll probably develop a cordial relationship with her. Good luck to you both.