This study examined the substance use and delinquency outcomes for the nearest age siblings of substance abusing and delinquent adolescents that participated in a randomized clinical trial evaluating the effectiveness of integrating evidence-based practices into juvenile drug court. The sample of 70 siblings averaged 14.4 years of age, 50% were male, 71% were African-American, and 27% were white. Measures of sibling substance use and delinquency were collected at four points in time (i.e., pretreatment, 4 months, 12 months, 18 months). Multilevel Longitudinal Models were used to evaluate whether changes in sibling substance use and delinquency paralleled the treatment effects observed for their substance abusing delinquent brothers and sisters in the juvenile drug court study. Parallel sibling outcomes were obtained for substance use but not for criminal behavior, and possible reasons for the divergence in these results were discussed. The findings add meaningfully to the literature on the effects of evidence- and family-based treatments on siblings. (Contains 1 figure and 2 tables.)