Stepdaughter???

I have a stepdaughter she is 7, she has lived in a home with my husband and I and her 6 year old sister and my two children ages 3& 4 for almost 4 years.We have the weakest relationship,as long as she cannot manipulate Dad into what she wants she is angry and detached from me,she hasnt seen her Mom in almost 4 years,she is on drugs and now on her way to do like 4 years in prison,the other child is as bonded to me as my own.I cannot imagine going through this a whole lot longer,feels like a battle zone alot of the time,I dont treat her any different than the others,they were so little and Mom wasnt around disrupting the bonding process so its like they are mine,I imagine there is some heartache having to with Mom,specialists say she uses that as a crutch to excuse her behavior.I am frustrated any ideas will be appreciated!!