Sassy pants!

There are some things that she should have a say in like she should be able to chose the clothes that she is wearing. Even if what she picks out doesn't match or she wears it inside out. There are other things that you should have the only word on like whether or not she can go to a frieds house alone. What you need to do is set limits. Let her know which things she can have a say in and which things she doesn't have a choice about. Make sure there are do-able consequences for talking back to you on those things that you have established are your territory alone. Do not engage in an argument with her. Follow through with the consequences. Be consistent. It will take some patience but you can overcome this problem

mine is a boy and i just tell him im grandma, not his mother! (which is my daughter) and i dont put up with his sassing me. i tell him how its going to be and ignore anything else he has to say. i dont yell like his mom. and he listens, i dont know why. i do threaten to spank. but havent had to. lol.

My daughter is sassy and says things that are exactly like something I would say. It is really weird. I get on to her but then I feel bad because I do the same thing. I just try to make her understand that there is a time and a place for everything and she is welcome to voice her opinion to me or her step father as long as she is not sassy us. My daughter is 8 and it seems to work, but I dont know how old your little one is so this may not be helpful.