Approximately 95% of Americans have at least one sibling (Weaver, Coleman, & Ganong, 2003), making it more likely that children grow up with a sibling than with a father (Lyon, 2009). It is therefore somewhat surprising to learn that sibling relationships have not been a central focus of psychological research, especially considering the fact that parent-child and peer relationships have been studied so extensively. There is no doubt that parents and peers have profound effects on children's emotional, psychological and social wellbeing, but siblings have important effects as well. By middle childhood, children spend more time with their siblings than they do with their parents (Pike, Coldwell, & Dunn, 2005). The sibling relationship is one of the longest and most lasting relationships that we as humans have. Approximately 78% of Americans over the age of sixty still have contact with at least one sibling (Cicirelli, 1995). Unlike parents, siblings are often in our lives until the end of our lifespan and, unlike friends, we do not choose them. They act as teachers, friends, and critics, just to name a few, and they are often a sounding board off of which we can test our ideas and behaviors. The focus of the current study is on the twin sibling relationship quality in middle childhood and in adolescence and its implications for individual adjustment, specifically in the realm of prosocial and aggressive behaviors. I evaluated twin sibling cooperation and conflict at both age 7-8 years and age 12-14 years and then examined prosocial and aggressive tendencies concurrently and longitudinally to study the strength of the association between the two. This study also aimed to better understand the extent to which prosocial behavior and aggression are influenced by genetic and environmental factors.