Need help with problem almost 13 year old girl

Im not a foster or adoptive parent, but my husbandҒs cousins daughter may need to come live with us full-time. She has some very difficult behavior problems and IҒm completely unprepared on how to deal with them. I hope any foster parents on here might be more experienced with kids with these types of problems or any recommendations for books or other forums. A little background: My husbands cousinҒs daughter (for short, Ill call her my cousin, but thatҒs how were related) is 12, soon to be 13. SheҒs been living with her grandmother (my husbands aunt) for about 6 months. Her mother is serving 3 years in prison, her dad (my husbandҒs cousin) struggles with mental illness and substance abuse problems. Her grandma is dealing with a serious illness that is getting progressively worse. My husband and I live in the same town so weve been trying to help her since the girl is causing so difficult and causing so many problems. I guess you could say we are doing respite care. We babysit some afternoons and have her some nights to give her grandmother a break and to see how things would go with her living with us - which IҒll admit is not good. Nothing has an impact on the girls behavior. Her grandma has tried behavior charts, reward systems, positive reinforcement, getting tough. She doesnҒt listen to my husband and I either and we feel as if we have absolutely no control over anything she does. The behavior problems keep escalating. Money has disappeared from her grandmas purse. She lies. She acts as if she canҒt hear you when you ask her to do things. She has cussed out just about everyone, including me, her grandmother and my 4 year old daughter. She has developed the habit while at our house of going downstairs while were asleep and watching tv. IҒve been looking through every book I can find on how to deal with these types of discipline problems and cant find any solutions that are going to work. IҒve downloaded Parenting with Love & Logic, The Connected Child and Raising Kids Who Refuse to be Raised on my Kindle, but Im not sure any offer a solution that will work for my cousin. A lot of parenting books are geared more for younger kids. But sheҒs not a teenager either. These books also are more for parents or adoptive parents. She also gets very defensive about anyone acting like her mom or trying to replace her mom. She is very clear that she doesnt want anyone to act like a parent towards her and I donҒt want to replace her mom either. Theres nothing to take away or use as leverage. She doesnҒt have a cellphone or Facebook account. She doesnt play video games. She doesnҒt have any friends or ask us to take her places like the mall or to parties. She doesnt want to do any activities at school. She doesnҒt care about clothes or makeup. She uses the internet to email her mom, but doesnt care if she doesnҒt have internet access otherwise. She doesnt listen to what we say nor does she care about natural consequences. She doesnҒt ask us for anything. If theres something he does want, she does behind our backs and does whatever she wants or steals. SheҒs been given the opportunity earn anything she wants as a reward for good behavior but that hasnt worked as motivation. We canҒt figure out how to discipline her. She ignores what we say. Getting her to do chores is next to impossible, so doing something as punishment hasnt worked. ThereҒs nothing to take away. She is seeing a therapist, but there are a lot of issues, mainly related to her mom, and her therapist hasnt been helpful in providing solutions for behavior problems. We understand it's going to take time, but things are getting worse not better. IҒm very concerned that I have no ability to handle an almost-teen with these problems and what will happen to her if her grandmas illness gets to the point where she canҒt care for her. Were the only close family in this area. If we wonҒt take her and her grandmother is to ill to take care of her, then shes going to end up in foster care. I know there are great foster parents out there, but there is also bad ones and her grandma especially is very fearful of what will happen if a relative canҒt take her for her. But I dont think I can deal with her behavior and I am afraid of the impact she will have on my two daughters. Any advice will be greatly appreciated.