Elana, I'm sorry you're 'trapped' on holiday with such a negative person! I think it's hard for most people to be around negative people, especially for a pro-longed amount of time. It's true, when you're around positve, happy people the attitude rubs off on you, and the same with moody, negative people. I can tend to be negative and I know when I am it affects the people I'm with. But when I'm happy they're happy.
I wouldn't advise you to think there is something wrong with YOU because you feel this way around her! I'd like to see anyone be around someone who is constantly moody and try to remain upbeat. It would be quite hard.
My only suggestion is perhaps try to have some time with just you and your partner.. even if just for the evening. It must be hard to tied to your partner's sister, it'd be good to spend some time with just your partner and do something you want to do - afterall it is your vacation too. If it's not possible... I don't know, just try to get through it the best you can, and be glad it's not a constant thing :P