Considering this was for a all-dialogue contest I actually got a really good feel for the characters. Sierra seems like a love-sick puppy, and her mother seems caring and like she's trying to nudge her daughter in the right direction. The dialogue was very realistic, and reminds me of how me and my mum used to be. The way you ended it was interesting, both characters seem exacerbated and frustrated with the argument, like it was going around in circles, like real arguments do.
Good job! :)