She is just pushing to see what she can get away with. I would suggest that you sit down and have a family meeting. During the meeting you need to set forth the rules and consequences. Let her have input into these things. If you just do it then it is just rules that the adults made up, when you let her have input then she feels that you are listening to her, respecting her and she will be more willing to follow the rules. The other thing is you must be consistent with both the rules and consequences. Example, Mon- she backtalks, consequence- time out. Tuesday- more backtalk, cons. none because you are too busy making dinner etc... If you have this approach then she will quickly learn that if your busy then she can get away with things. If you remain constant with the rules-consequences then she will learn that if she does this action this will be the consequences.
My youngest son has a best friend that when he wants something he will just keep asking until he gets it. He wanted my son to come up to his house but my son did not have his room cleaned up so he could not go. The friend consistently called our house asking me if my son could come up yet. After 3 phonecalls in 3 minutes, I told him that if he called again my son would not be allowed to come up at all. He said ok- 5 minutes later he called. I stuck to my guns and did not allow my son to go up that day. The next day his friend came to the door and asked if he could play. My son had his chores finished so I let them play. He thought that my son just did not want to play the day before, my son said no, Mom would not let me come up because you kept calling, he could not believe it so he came and asked me, I gave the same answer. His reply was "Wow, at my house I just keep asking until my parents let me do what I want. Can't your son do the same thing?" My answer was no, I told you that if you called again and you did that he could not play. That is what happened. So if you want your children to follow rules all the time then you must be consisitent with it. Good Luck!