Depression stuff. I completely understand, that's what happens to me. I'm waking up every morning these days determined to stay happy, no matter what's happening. Its going pretty well actually :)
I think you just need to find a moment to take a break and relax. The more things you start worrying about {including worrying, and worrying about worrying) mount up and you'll just end up having a lot of rapid break down's.
My old best friend was one of the most competitive people you will ever meet. She had the biggest problem if I happened to be confident with anything. Its not healthy. You just need to realize that your sister is trying purposefully to make you upset, whatever her reason is.
If you need anyone to talk to, I'm always here. And I'm quite serious, even if its just a little "Hi" or a full-blown rant.