We tested social comparison predictions about cross-sectional and longitudinal associations between parents' differential treatment of siblings and both youth depressive symptoms and sibling relationship qualities from middle childhood to late adolescence, controlling for dyadic parent-child relationships and siblings' ratings of parents' fairness. Participants were parents and first- and second-borns (M= 11.8 and 9.2 years old at Year 1) from 201 White, middle/working-class families. Three-level models revealed both cross-sectional and longitudinal linkages between differential treatment and outcomes. For example, youth whose parent-child relationships decreased in warmth relative to those of their sibling reported increases in depressive symptoms and decreases in sibling warmth. Gender and age moderated differential treatment-depressive symptoms associations; birth order moderated differential treatment-sibling relationship associations.