10 year old stepdaughter refuses to listen to me?

I have been married for almost 2 years now and my 10 year old stepdaughter refuses to listen to me. She is defiant and ignores me. I ask her to do something and the response I get is why do I have to? I could tell her the sky is blue and she will argue with me that it is not........I try to talk to her mother about her behavior towards me but all I get is an attitude and I am told that I am "gunning" for her daughter. I do not know what to do, it is to the point where i sleep on the couch and the relationship is straining to the limit. I asked my 10 year old stepdaughter why she does not listen to me, the response I got was " I hear you, I just choose to ignore you" Seriously, does anyone have some advice? I am at my wits end with this child. I have 2 grown sons and they were raised to respect adults, This child has no respect at all..............Thank you for reading this.