8 year old acting out

The child is an 8 year old girl.  Her parents have been divorced for almost 3 years.  The girl originally lived with her mother, but her mother got into drugs and other things.  The mother gave the girl to her father and eventually signed over custodial rights.  Currently, they have joint custody, but the girl lives full time with the father.  For two years, the mother did not spend much time with the girl.  She was supposed to get her every other weekend, but did not do so the majority of the time.  During that time period she got in trouble with the law and is currently on probation for larceny charges.  She probably only saw her a roughly 10 times between the two year period....until the father started seeing someone else and moved in with her and her two children, a 12 year old girl and 5 year old boy.  They have lived together since March 2007 and got married in Oct 2007.  Immediately after they moved in with his now wife, the girl's mother starting seeing her every other weeked. The girl's mother gets upset with the father and accuses him of not letting her spend time with the girl, even though he has never done anything to prevent her from getting the girl on her scheduled weekends and even agreed to letting the girl and the mother have dinner together every other Wednesday night.  On occassion, the mother does not bring the girl home until after 10pm on Sundays (school nights) and does not allow the father to talk to the girl during the times that she has the girl.  In addition, the girl complains often that the mother does not spend a lot of time with her and cries frequently on Sundays when she returns saying that she didn't get to see her mother much.  The new wife and girl get along well and the girl even refers to her as "mama"; all three of the children get along well, with occassional disagreements that would be expected between siblings.  The new wife's children like the girl's father as well.  There have been a few disagreements between the new family members, but for the most part they have all seemed to be settling well...until recently.  The 8 year old girl has recently started acting out.  It started with a few marks on the walls with a permanent marker and taking various items out of the older girl's room and hiding them.  It progressed to taking the older girl's homework assignments and destroying them.  Next, she started writing on the frames for the wedding pictures and hiding sentimental items from the wedding.  Now it has progressed to writing "I hate you" on the walls in the hallway and the older girl's room and the door to the master bedroom. When asked why she is doing these things, the girl responds that she wants to live with her mother because she misses her and she thinks that if she gets in a lot of trouble she will get sent to live with her mother.  The father does not feel that allowing the girl to live with her mother at this point would be beneficial to the girl.  The mother does not have a job, does not have a permanent home, and is currently on probation - not a stable environment for any child.  This family has the potential to be very, very happy together.  I think the parents are at a point of not knowing what to do.  They have tried taking privelages from the girl, grounding her, and putting her in time out.  However, this is an every day occurance and it seems that nothing they are trying is getting through to her.  They have an appointment to meet with a therapist to find out what the underlying problems are and how to fix them, but the appointment is not for another week.  Do you have any suggestions for things they may be able to do to try to help calm things down until then?  They need help!!