Siblings can often be seen playing the same sports and participating in the same physical activities. The Williams sisters in tennis and the football-playing Manning brothers are well-known examples of siblings in professional sports. More than 80% of children in the United States have at least one sibling (Horn & Horn, 2007) and the prevalence of twins has increased significantly in the past few decades. Given the overwhelming majority of children who have a sibling, and as sport continues to be a valued activity in many American households, more and more children are participating in physical activities and sports with their siblings.