This study examined within-family stability in parents' differential treatment of siblings from adolescence to young adulthood and the effect of differential treatment in young adulthood on grown siblings' relationship quality. The author used longitudinal data on parent–child and sibling relations from the sibling sample of the National Longitudinal Study of Adolescent Health (N = 1,470 sibling dyads). Within-dyad fixed effects regression models revealed that the adolescent sibling who was closer to parents went on to be the young adult sibling who was closer to and received more material support from parents. Results from an actor–partner interdependence model revealed that differential parental financial assistance of young adult siblings predicted worse sibling relationship quality. These findings demonstrate the lasting importance of affect between parents and offspring earlier in the family life course and the relevance of within-family inequalities for understanding family relations.