Sibling interactions are natural contexts for learning about the appropriate expression of emotions. The emotionally charged nature of sibling interactions creates a convenient context to explore emotional reactivity and regulation. The purpose of this study was to examine the relations among parent-reported sibling relationship quality, observed sibling prosocial and antisocial behaviors displayed when playing a competitive marble game, and children's emotions coded from videotape. The sample consisted of 58 twin children who are currently participating in the longitudinal Arizona Twin Project. Parents completed the Sibling Relationship Questionnaire online at 5 and 8 years. Additionally, a competitive marble game interaction between the siblings took place in the home at 8 years and was videotaped for objective coding of prosocial, antisocial, and control behavior. Facial expressions were also coded from videotape using Emotient FACET software across the marble game interaction. Three mean composites of emotion were created, including positive and negative emotional facial expressions. Results showed that parent reported warmth did not predict the occurrence of positive emotions during the sibling interaction. However, siblings with high conflict showed less fear during the interaction. Parent reports of warmth predicted the extent to which siblings differed on emotion expression, however conflict did not. Parent ratings of conflict and warmth did not predict the extent to which the sibling dyad was emotionally intense. Findings regarding genetic and environmental effects were in line with previous reports of genetic influence on prosocial behavior and negative emotion, and expressions of joy being influenced by the environment. This study investigated noteworthy aspects of the sibling relationship that appear to promote children's adaptive development.