Similarities and differences in adolescent siblings' free time activities were investigated. Firstborns and second-borns from 181 predominantly White, working, and middle-class families reported on their time use and sibling relationships. Their parents reported on their socioeconomic status and neighborhood characteristics. Cluster analysis identified three groups of sibling dyads: Cluster 1 differentiated in leisure and time with friends, Cluster 2 differentiated in instrumental activities and time alone, and Cluster 3 did not differentiate in time use. Multinomial logistic regressions revealed that the longitudinal links between sibling relationship qualities and cluster membership were moderated by firstborns' sex. Findings highlight the importance of person-oriented analytic strategies for understanding sibling similarities and differences and of within-family variability in youth's time use.