I'm so upset, I wish my sister was never born.
That sounds awful, why would you wish that?
Because of her I'm no longer an only child. My parents always love her more.
I think it's okay to FEEL that way sometimes. But be careful expressing that thought, especially to your sister. I'm sure you can imagine how hearing that may be devastating to her.
I don't care if she feels hurt, my feelings are hurt as well.
It sounds like you should talk to your parents about how you feel and explain that you're feelings are hurt. Don't take it out on your sister, she may not even understand why you have these feelings towards her.