I have wondered this myself. My brother and I, as teens, fought ONCE. After that we have NEVER even argued. Always allies. Always there for each other.
Yet, I remember watching friends' brothers and sister act so evil to each other.
MY ANSWER: After wondering for years, and watching how they treat each other now, I've concluded that it is mostly a behavioral result of something toxic in the family. Perhaps bad parenting, perhaps private trauma a sibling experienced, or just some bad DNA.
DO know that, how you respond is MUCH more important than why they act like pr*cks. In one word: RESPECT. Respect yourself first, then them. Always. Do that and you will know how to respond to their crappy behavior. If nothing comes to mind, then just view them as having a very toxic, sickly disease. Hope they get better but avoid them, as you would if they were vomiting, sweating, shaking with a disease. They are.
No one can choose who their siblings are. Only how we ourselves think and act.
Final note: A human's frontal brain (that is the part that makes us smarter than other animals) is not connected to the rest of the brain until around 25. So if your brother or sister treats you bad, view them as an unpredictable animal. In a way, they are. And until 25, so are you.