Siblings will fight at some point. It's the nature of living with someone in close quarters. However, there's a difference between sibling rivalry and being cruel to your sibling. Some lines shouldn't be crossed.
I have a brother who is two years older than me. He has suffered from a condition since birth that forces him to be in a wheelchair anytime he is out of bed. He cannot use his arms that well, either. We used to have a great relationship until I turned 11, and I really started resenting the amount of attention he took from my parents. And how much stress he put them under. I had to help him, change him, and feed him constantly. I hated it.
There were a few years in his life when he was so mean to my parents, and it really made me resent him more. I realize now that I had no right to judge him, and I will never know how difficult life is for him. I stopped hanging out with him and made more friends at school. I would tell my family about all the fun I had at school during dinner to make him feel bad.