The sibling relationship is a natural laboratory for young children to learn about their world.3 It is a safe and secure place to learn how to interact with others who are interesting and engaging playmates, to learn how to manage disagreements, and to learn how to regulate both positive and negative emotions in socially acceptable ways.33 There are many opportunities for young children to develop an understanding of social relations with family members who may be close and loving at times and nasty and aggressive at other times. Further, there are many opportunities for siblings to use their cognitive skills to convince others of their point of view, teach or imitate the actions of their sibling. The positive benefits of establishing warm and positive sibling relationships may last a lifetime, whereas more difficult early relationships may be associated with poor developmental outcomes. The task for young siblings is to find the balance between the positive and negative aspects of their interactions as both children develop over time.