Step-child issues!!!!!!!?

Pleases I need advice on how to be a step-parent to a 8 year old mixed girl who thinks the world revolves around her,and that the world owes her everything. Said child has NO respect for me, and really not even for her own parents. If we tell her to do something ,anything, she gives you the eat ****and die look. Her dad has told her I am not going anywhere, that I will be in her life from now till eternity. Hes also told her that I being the grown up have the right to tell her what to do. he also tells her that I would never tell her anything wrong to do. Said child comes to visit on the weekends from friday night to sunday night, the whole weekend she finds every way imaginable to cause trouble. Then when I go to correct her she huffs and snorts at me. In return I amd the one who gets fussed at by her dad like I'm the child. I am tired of getting in trouble because she is a BRAT!!! Another thing she loves to do is go behind what I say to get her way. Thanks for the help