Bidirectional associations between sibling relationships and children's problem behaviors are robust, and links with prosocial behavior have also been reported. Using cross-lagged models, we were able to conservatively test temporal directions of links between positive and negative aspects of sibling relationships and children's prosocial behavior and conduct problems across a 3-year time span in middle childhood. The Avon Longitudinal Study of Parents and Children (ALSPAC; http://www.bristol.ac.uk/alspac/researchers/data-access/data-dictionary/) is an ongoing population-based study designed to investigate the effects of a wide range of factors on children's health and development. For the purposes of the current analyses, we included 2,043 ALSPAC families who had just 1 older sibling as well as the target child, with an age gap of no more than 5 years. Mothers reported about the quality of the sibling relationship and both children's prosocial behavior and conduct problems when the target child was 4 years of age and again when the target child was 7 years old. Confirming our hypothesis, individual child behavior was predictive of sibling relationship quality, and sibling relationship quality was predictive of later child behavior, providing robust evidence of bidirectionality for both prosocial behavior and conduct problems. It would be consistent to expect that an improvement in either sibling relationship quality or individual children's behavior could have a positive spill over effect. We also found evidence of older sibling dominance in the domain of prosocial behavior and the positive aspects of sibling interaction.