I wrote this review last night (when there was only one chapter up!), so it is already sort of out of date....
A very good beginning to your story -- not so long that it repels readers, nor so short that it leaves them impatiently waiting for more, and, might I add, I very much enjoy reading Draco/Hermione fanfics (although I am extremely irked by people who think this "ship" could actually happen!

). I also like it that you have created your own new character and have successfully maintained J.K. Rowling's writing style in your characterization and narration.
In my opinion, this initial description of Carmen is what makes this fanfic so brilliant. The single paragraph sets up all of the forthcoming action because it reveals crucial aspects of Carmen's and Hermione's personalities that (if my divination proves correct) will be central to the story's plot. Hermione's observation that "the only thing [she] possessed that Carmen didn't was magic" reflects the insecurities that we frequently see in her character, particularly in the first half of "Sorcerer's Stone." While Hermione is not necessarily a jealous person per se, certain scenes of the first book (e.g. when Snape is asking Harry all the difficult potions questions and Hermione stands up and waves her hand in the air to answer) demonstrate her reliance on a few particular talents to stand out in a group. It seems to me that she similarly relies on her magic to earn her family's recognition, and this one distinguishing factor has just been taken away from her by Carmen, the mature, attention-attracting younger sister. This paragraph is an excellent set-up for the rest of the story.
Then we see another rather cunning, devilishly tricky dimension of Hermione's many-sided personality as she accepts Carmen's dare and dresses to go meet Harry and Ron in Diagon Alley. I remember reading another fanfic about Hermione and a dare on Veritaserum at some point -- I think it was whomeisi's "It All Comes Out in N.E.W.T. Potions Class," which isn't on the site anymore -- and these stories can really be quite fun. As we know, Hermione is not one to run away from a challenge -- just look at her punching Malfoy in "Prisoner of Azkaban."

This little Hermione scene is particularly good when juxtaposed with her earlier somewhat-insecure feelings. Although this is only the beginning of the fanfic, I'm already seeing it as a story about Hermione's multi-layered personality as much as a story about romance. Stories that are entirely "shipping" can be rather boring, so I think this is a good sub-plot. Or perhaps I am reading into this story too much entirely.

If that's the case... well, I've always believed that the reader's interpretation of a novel is much more important than the author's intention...
After this positive review, I have one rather picky suggestion to make. I bother nearly everyone about this (just ask Esrb99 and some other fanfic authors on Veritaserum!

), so I feel burdened to pass on my message to you as well: begin a new paragraph with each new speaker! It'll make me very happy if you go back into your original post and edit this mistake....
Overall, a very good first chapter! I'll try to review the next chapter sometime later this week if you'd like.