The talking back stages

Well we have finally hit the age where my oldest daughter believes it is okay to talk back to her parents or assume she is right in a situation where she is clearly not. How do you explain to a 6 year old effectively that talking back in unacceptable and will not be tolerated? Because apparently I have the least bit of an idea. My husband and I have had sit down talks with her about how that is unacceptable behavior and my ex husband says he has had this discussion with her however him being him he swears that it is all my fault because I let her think she is right when she is not. I have only allowed to to be right when she wasn't once an that was because she was mid meltdown over an advent calendar that my mother made her for Christmas. I know I am not supposed to feel this way, but I feel as though she has been taught that it's okay to be disrespectful towards us when she is at my home. Who knows maybe it is my fault and I was too soft on her when she was younger and now her being six she is using that to her advantage. All I know is we need to nip this in the butt, because back talking, eye rolling and whispering under her breathe is not okay and I will not allow her to do so. I didn't see this phase coming so soon. I figured maybe when the teenage hormones began to flow through her that then we might run into this typw of behavior, but never did I imagine it would start at age six. I am at a loss, I admit I try not to be too harsh on punishing the girls as I want them to know that mommas house is a safe place because myself and their father do punish and parent differently. I suppose I will have to become firmer with her and bring back the naughty corner. And begin having her explain to me why she was punished and what behavior she is not to continue and see if that does the trick. For all the parents out there going through the same troubles I'd love for you to leave me a comment with any suggestions you may have. I'm just a regular ol momma trying to raise her two crazy girls and make co-parenting work while not driving the love of my life insane in the process. He handles this all so well and I don't know how he takes on so much and does it all like a pro, however I won't question any of it because clearly it is in my life plan to have him by our sides and I couldn't be more appreciative of how supportive he is of me and the girls. Getting through this crazy age of six is a lot harder then I thought it would be. Everyone always warned of about how I have two girls and it's gonna be so crazy when they are teenagers. Well why in the world did they not warn me about age 6! Forget the teen years there's a lot more to take from them to correct behavior at that age. Plus that's seven years away and I'm hoping it will go by slowly so my baby girls stay my babies for a few more years. My four year old is so sweet and respectful and adorable, their personalities couldn't be more different, but then again they always have been. But regardless even if I end up on the brink of insanity I will still strive to keep my chin up, eyes forward and take this parenting my attitude ridden child head on because at the end of the day she is still my little princess and I'm sure at some point my mom had to put my sassy tone into place as well. So until tomorrow everyone have a great day!