"I know they seem very womanly but they are children in their heads."
THIS
I was a 15yo SD once, I think remembering this will really help you.
Also, it's OK to be annoyed with her when she acts out, and it's also OK to tell her that her behaviour is unacceptable and what you expect of her.
You have to balance these criticisms out with positive interactions though, tough as I know that will be, especially to begin with! If you make an effort to be friendly and enjoy time together in the face of her tantrums, she'll learn that you aren't the enemy. Eventually.
You will have to be persistent, consistent, patient and forgiving for a bit though. You are the adult, so the onus is on you to keep turning the other cheek in the face of some seriously unreasonable behaviours.
Basically teenagers = terrible twos with hormones.
Stepchildren who are teenagers = terrible twos with hormones and lots of family baggage.
I also recommend the book "How to talk so kids will listen and listen so kids will talk." It has a lot of useful teenager strategies.
I know it's tempting to not bother to maintain relationships with stepchildren who are teenagers too much, cos you figure they'll be out the house in a few years anyway and they're SUCH a PITA.
Thing is, she'll be in your life as long as you are in a relationship with her father, which I'm presuming you want to last a long time. Prevent her being an issue between you by fostering good relations NOW.