I find that clear written rules are the easiest to follow. Make some poster board rules and post them in a none public area like the laundry room and have just a few, like:
Curfew is at XXX and if you are late XXX are the consequences.
We expect you to clean your room every XXX and if you do not do this XXX are the consequences.
Homework, activities, manners, all kinds of things could go on the lists. But it should be simple and if you take time to communicate even if something comes up you had not thought of then talking about it and deciding on a consequence then may be more helpful.
She already has a step mom ordering her around and she is needing a patient, compassionate person to guide her to maturity and adulthood right now.
One thing that will come up is guardianship. Biological parents can revoke it at ANY TIME they so choose but if you have this girl and she needs to enroll in school, play a sport, gets hurt at school, etc...you will have to have legal authorization to act in her behalf.