The present study examined the moderating effect of the quality of the sibling relationship on the longitudinal association of parental treatment with theft, vandalism, and violence in adolescence. Participants were 416 sibling pairs which were studied over a one-year period. The younger siblings were aged 13 to 15, the older siblings 14 to 17 at Time 1. No significant effects were found for mixed-sex dyads. For same-sex dyads, the results suggested that when the relationship was of poor quality, younger boys who felt treated less favorably by their mothers were most likely to show high levels of vandalism and violence, while younger girls who felt treated less favorably were most likely to show high levels of theft. No such effects were found for older siblings. These findings indicate that differential parental treatment and the quality of the sibling relationship have gender-specific effects on adolescents' delinquency and have a different meaning for younger than for older siblings.