Sibling collusion is a process by which siblings form coalitions that promote deviance and undermine parenting. Collusive sibling processes were identified and measured using macro ratings of videotaped family interactions. Hypotheses were tested on a multiethnic sample of urban youth, with a target child identified as either “high risk” (n = 26) or “normative” (n = 26), and their families. Siblings in families with a high-risk target child showed reliably higher rates of collusion than those in families with a normative target child. Sibling collusion also accounted for variance in problem behavior after controlling for involvement with deviant peers. Findings suggest that deviant conduct forms a common ground among siblings, potentially amplifying risk of mutuality in problem behavior during early adolescence. These data also indicate that attention to sibling relationship processes is relevant to family interventions designed to mitigate the development of behavior problems.