Having a sibling provides an opportunity to learn to get along with others. Especially when siblings are younger, they may fight bitterly, but they will probably be playing together again an hour later. For example, a child will say something hateful to a sibling, knowing full well they will still be siblings and friends when the fight is over. If the same thing was said to a playmate outside the family, that playmate might take his or her marbles and go home for good. Thus, children learn from relationships with siblings just how certain words or actions will affect another person without the fear of losing the person's friendship.
Siblings fight for a number of reasons.
They fight because they want a parent's attention, and the parent has only so much time, attention and patience to give. They must love him more than they love me. Children need not weeks or months but years to learn some of the socially approved ways to behave in relationships. Lessons about jealousy, competition, sharing and kindness are difficult to learn, and, indeed, some adults still haven't learned them. Adolescents fight for the same reasons younger children fight.