It doesn't sound as though you do feel terrible admitting you hate your sister, Sarah. I agree with B00; that there are no rules stating you have to like everyone you're related to, but if you really don't like her as much as you say, then why don't you keep away from her? It's obviously not making you feel great (or her, probably) and there's no forcing yourself to like anyone.
There's only 21 months between my own sister and me, but we couldn't be more different. I love her to bits, but she aggravates me beyond belief. My sister is bubbly and gets along with everyone, whereas I'm far more private.