Existing research on aggression tends to narrowly focus on peers; less is known about sibling aggression, most likely due to its historical acceptance. Aggression is characterized by its forms (i.e., physical vs. social or relational aggression) and its functions (i.e., the motivations behind the aggressive act and categorized as proactive vs. reactive aggression). We use data from a two-wave study of middle (n = 197; M age = 12.63 years at Wave 1) and older (n = 159; M age = 16.50 years at Wave 1) adolescents to assess the extent to which proactive and reactive functions of sibling aggression make unique or conditional contributions to adolescent adjustment (i.e., depression, delinquency, and substance use). We find that proactive sibling aggression increases risk for problem substance use and delinquent behavior, reactive sibling aggression increases risk for depressed mood and delinquent behavior, and such results are observed even with statistical adjustments for sociodemographic and family variables, stressful life events, and prior adjustment. Few conditional effects of proactive or reactive sibling aggression by sex or grade are observed; yet, for all three outcomes, the harmful effects of reactive sibling aggression are strongest among adolescents who report low levels of proactive sibling aggression. The results speak to the importance of understanding the proactive and reactive functions of sibling aggressive behaviors for adolescent adjustment.