Results indicated no significant impact of parental differential treatment and the quality of the sibling relationship on delinquency involvement for mixed-sex dyads. On the other hand, for same-sex dyads, the findings indicated that when the sibling relationship was poor, younger boys who perceived less favorable treatment by their mothers were more likely to display high levels of vandalism and violence. Younger girls who perceived less favorable treatment by their mothers were more likely to exhibit high levels of theft. No effects of parental differential treatment and quality of the sibling relationship were found for older sibling pairs. Future research should include a more diverse population sample as well as an examination of sibling behavior. Participants were 416 Dutch families which consisted of a mother, father, and 2 adolescent children. These families had 216 same-sex sibling pairs and 200 mixed-sibling pairs. Each family member completed a questionnaire in their homes during two points of contacts spanning a 1-year period. The questionnaire focused on perceived differential parental treatment, parental warmth, quality of the sibling relationship, and delinquency. Data were analyzed using a series of hierarchical regression analyses. Tables, figures, references