Siblings play a critical role in children’s behavioral development; yet sibling-focused assessment and intervention for youth behavior concerns are uncommon in social work practice settings. To address this research-to-practice gap a scoping review of research focused on siblings and the development of externalizing behaviors in childhood and adolescence was conducted. Forty-three empirical studies published between 1997 and 2017 were reviewed and synthesized. Results illuminated a number of processes through which siblings influenced the development of externalizing behaviors. Identified behavioral domains included conduct problems, substance use, and sibling abuse. Sibling negativity and hostility, coercive sibling interactions, and sibling collusion were consistently associated with the development of conduct problems. Substance use behaviors were primarily influenced via sibling role modeling, social reinforcement, facilitating access, and co-use. Moderating effects of parental involvement, peer influence, sibling age range, and sibling gender composition were also observed. Less research was conducted on the processes underpinning sibling abuse, although prevalence studies suggest high rates of sibling victimization, particularly among close-age siblings and male-male dyads. Results indicate the need for social workers to consider both the characteristics of sibling groups and the quality of sibling relationships when assessing and intervening to prevent and address externalizing behavior problems in children and adolescents.