My 8 year old step daughter

I have been in a relationship with my fiance for 31/2 years. In September we won temporary custody and are fixing to be going back for full custody. We have had the kids living with us since April of last year. There are 3 kids. The ages are 6, 7, and 8. The oldest is the one that I am having the most trouble out of...She is so rude. She has a bad attitude towards me. She use to not until her mother told her everything that everyone said when we went to court. Now she doesn't want to do anything I tell her to. She is always telling me when she has to be sent to her room that she is going to tell her mom so that she can tell the judge. I have tried time out, sending her to her room, making her go to bed early, taking priviledges(sp?) away, and even spanking her. I hardly ever spank. I know that they are going through a lot and I try so hard to be able to get a long with them and be there for them but when it comes to the oldest nothing I do is working. We have been going to church lately and there is a lady that she really likes and it just tears me up cause we use to be like that. Last night she got so mad because she had to come home and couldn't go with that other lady. I really think that she hates me. I know that she resents me for being with her dad. I just don't know what to do with her anymore. We have had alone time and she loves it when she can be alone with me but with 3 kids we can't do that all the time. I know that her mom doesn't like that fact that I refer to them as my girls. I don't do it in front of their mom but to my friends and stuff I do call them my girls or my son. There are 2 girls and one boy. I know that she says stuff to the kids about how the aren't my kids and just gripes. I don't blame her in a way but I am by no means trying to take her place but they do need a mother figure and they only go to their moms once during the week and then every other weekend. I am trying so hard but like I said the oldest just has a very bad attitude towards me. I don't know what to do I could use some advice.

[Encephalomalcia]

She will resent you and wish that her parents were together like all kids of divorced or separated parents do.  Age 8 is a hard age in itself without bringing in changes that these kids are having to deal with.  Lots of patience.  When you punish them, make sure that it is something of punishable size.  Don't sweat the little stuff, that will only add to the resentment.  Our job as parents is to guide our children to help them make decisions on their own and not to control them.  Make sure that she is having some control like picking out what she wants to wear, etc.  Give her lots of choices in everything that you do.  Children love to feel that they matter and giving them choices helps them feel that they do.  It makes them proud.