I want to stop hating my sister but I can't?

My sister is seven years younger than me. Ever since she was born I've been unhappier. She has always been very sickly and my parents and grandparents have devoted a lot of time to her and have always constantly doted on her, insisting that she needs it because she's "sick". She is all they can talk about, and every time I talk to them they bring her up and talk about how they are so sad about how sickly and poor she is. She is extremely annoying and selfish and bratty, and she literally acts like a five year old (even though she is much older than that). My parents' justification for this is that she has been too busy recovering from illnesses all her life to have time to mature and develop as much as she should. They always take her side for things. She always expects to be the center of attention and I absolutely hate her. Just looking at her makes me seethe with anger, so I snap at her all the time. It's the very fact that she exists that makes me hate her so much. I have no love for her at all. I've tried to feel it but I can't. I know this is horrible and selfish and I don't want to feel this way but I can't help it. What do I do?