A lot of animals fight with their brothers and sisters when they are young to practice hunting and other skills that they need to survive in the wild. I’m not sure humans have that excuse though. I think some of it is playing, some of it is because even though you love people, they still get on your nerves at times and you need to let it out.
Scientists also think that we use our families to practice social strategies that we need to use as adults to function in our communities. So we fight with our brothers and sisters because we are learning how to deal with anger, conflict, competition, jealousy etc. The older you get, the less likely these feeling will result in fighting and instead you will learn to talk things over and come to some understanding. Its a good thing that we learn to do this with our siblings, because we love them deep down its not as if they will hold all these fights against us. So its all pretty healthy really, so long as you grow out of it and are still not fighting when you are in your thirties!