Extremely disrespectful 7-yr old daughter

[andrewh81]

My 7-yr old daughter is extremely disrespectful, and sometimes physically violent with my wife and our other children.  I have a 9-yr old daugther, and an 18-month old daughter, and a baby on the way.  The only ones that my daughter seems to target is the 9-yr old, and when she does she often is verbally abusive and physically violent...sometimes for no reason.  She also gets extremely disobedient and disrespectful with my wife, and will often times whack at her or try to slam the door on her.  I just got a call from my wife saying that she laid out some clothes for her to wear, and my daughter responded by saying, "I'm not wearing that and you can't make me.." and then in a very sing-song voice she continued to say, "You can't make me, you can't make me....".  What makes all this hard for me is that this pretty much only happens when I am not home, and when I am home she does act mean to her sister once in awhile, but never to my wife, but for the most part she is very loving and caring.  So it is like two different people.  She is a very loving and sweet girl, probably more than her sisters, but her behavior is getting worse towards my wife and her sister and needs to stop.  CAN YOU HELP????