M.C.
Try reading this book: Raising your Spirited Child by Mary Sheedy Kurcinka. It may help. It also sounds like she really really needs consistency more than the other children. It is difficult on you and the child when you "try everything" because the limits of acceptable behavior may seem like they are moving around, resulting in more testing behavior. You really need to decide on what behavior is acceptable/unacceptable, communicate that behavior to her, along with consequences for each action. Then FOLLOW THROUGH ABSOLUTELY with consequences immediately, even if it messes up your schedule, she is late for school, or whatever. And it will get worse before it gets better - she will test you. It is so important that she knows where the limits are and what happens when the limits are tested. You may want to look into parenting classes in your area. I live in Hawaii and DOE offers FREE parenting classes that are excellent (and you bring your child, too). There are also classes offered by The Institute for Family Enrichment. My children and I have greatly benefitted from these resources. Good Luck.