Excellent story! There's no way in hell that this is your first one-shot. :p
Really, though, I loved the characters and the oddness of Kara's family. They each had their own personality, though they were all so bubbly and happy that they sort of blended together a bit, but in that situation, it's understandable. I found them all to be highly entertaining.
I don't have many negative comments to make about this piece. There are a few missed commas, a few missed words, a bit of awkward and/or redundant phrasing (For example, "I silently screamed inside" is too much; "I silently screamed" or "I screamed inside" would be just fine. Oh, and "I leaned against the ledge of the bridge and looked up to the sky, framed by the Big Ben." I think the sky would be framing Big Ben, not the other way around.), but there's really nothing major to nitpick, and considering that you're not a native English speaker, I can't even really fault you for those.
Overall, very nice work! I hope you kick ass at that contest. :)
And very different from some one shots I've read.