The moment I heard "I'm going to a birthday party!" I knew you were doing lines from the Digimon movie! I love that movie so much, I grew up watching it, so I really love how you were able to showcase the emotion from this scene so well!
My only negative thought on it is the overused and unfitting background music. I feel like you could have used something else that would fill up the silent spaces much better, and still flow with the mood of the situation. I feel like the music is a little too... "silly goings-on in a convenience store" feel, rather than embarrassing letter being sent by annoying little sister.
I do really like the transitions in the voices, though. Did you record all the lines for one character, and then the other? Or did you record one character's line, then the other character's line, then the first's line, rinse and repeat? I'm interested in voice acting, as well, so since you seem to know what you're doing, I'd love to know your secret!