This is a familiar situation, and so is the result.
You need to get your partner on side with you, and agree a future plan of action.
Have a serious talk with him about the situation, and what you are both going to do about it.
Rule One is - singing from the same hymn sheet. If you try and discipline this child - even in the sense of saying 'No', her immediate reaction will be to go to her dad. he MUST back you up at all times, even if he thinks you are in the wrong, and you can discuss it afterwards out of the children's hearing. This is standard behaviour, all kids do it, it's just more frought with step-children, but you have to be united, or she will play you off against each other.
Try and encourage her to bond with you - do 'girly' things together, shopping, DVD's and so on, and try to make she has some individual attention from you. This will overcome jealousy for your son who obviously has your main attention. It's a balancing act, but it can be done if you are strictly fair about it.
Make sure all the children have the same rules about behaviour, and they are enforced with scrupulous fairness. Of course you over-compensate with your son, and that will make him unhappy and confised as well.
Children respond to firm guideleines and good routines - put them in place for al of them, and things will improve.
It can be done, but it takes patience, and support from both of you, for each other.
Good luck.