Best Friend's sister is causing a problem?

My best friend, Melanie and I are both 14. We entered a dance competition this summer and got through to the finals. The finals arent locally - they are 3 hours drive from my house. Mel's parents agreed to take us there. The problem is, Mel's younger sister is coming too. Jess is 6 years old and she is really spoilt. Mel does everything what Jess wants her to do. If something doesnt suit Jess the slightest bit, she cries and her family do everything how Jess wants is Jess cries. My parents cant take us to the place where the finals are. The finals are this autumn. Because Mel's parents are taking us there, Jess will come too. Jess always throws strops and when we are there we wont be able to rehearse or do anything because Jess will throw strops that Mel is spending time with me instead of playing with Jess. So we will never have time to rehearse. I really dont want Jess to go. Mel loves Jess and will want her to go. Mel always says that Jess wont whine and throw strops, but then Jess does it. What should I do?