I secretly hate my sister for being skinnier than me
Would anything change in your relationship if you told her how you felt?
Nothing really, that would be normal
We don't have to like everybody but you shouldn't hate people for how they look. A part from that it is something people can't help sometimes, it makes you seem like a bully.
I'm going to post a tweet of her saying she's skinnier than me.
That is mean to keep talking about your sister that way because of her appearance. You shouldn't hate her because of the way she looks. You should try and find more confidence in yourself instead of comparing yourself to others.