Researchers from the University of Missouri have been looking into the thorny issue of sibling rivalry. They carried out a number of interviews of pairs of siblings between 8 and 20. They found that there were two types of conflict between siblings. The first kind was about equality and fairness issues such as taking turns and sharing responsibilities and these had no impact on relationship quality. The second type of conflict was over personal space, for example borrowing things without asking or hanging around when older siblings have friends over. When these conflicts were common both younger and older siblings reported less trust and communication.
You can find out more about this research at