My BF's sister is a problem?

My best friend, Melanie has a sister called Jessica. Both me and Melanie are 14 and Jess is 5. Mel and her family moved to Germany 3 years ago and they come to visit us 3-4 times a year. This summer Mel and I entered a dance competition and won a place in the finals which will be this autumn. Mel will come to England especially for it. It isnt locally, you have to drive for 3 hours and my parents cant take us on those particular dates, so Mel's parents said they would take us. The problem is, Jess is coming too. Jess is the youngest child in her family. She always gets whatever she wants and usually, when Mel and I spend time together, Jess gets angry and cries because she thinks Mel should spend all her life playing with her. When Jess cries, everyone does what she wants. She has learnt - if you want something, you have to cry. Now Jess is saying that she wants to be in our dance routine. She is rubbish at dancing, so she will just ruin it. Also, we got the place in the finals, and the rules are that we cant change the people in our group. I am worried she will ruin our time there, what should I do to stop her from going there? What should I do to make sure she doesnt come with us to the finals? Maybe so that she stays home with one of her parents or something!