The goal of this study was to compare developmental changes in adolescents’ relationship experiences with their siblings versus with their best friends and to determine whether individual differences in adolescents’ sibling experiences were associated with differences in friendship experiences. Participants included 179 firstborn-secondborn adolescent sibling pairs (N = 358). At the onset of the study, firstborns averaged 15 years of age, and secondborns, 12.5 years of age. Siblings were interviewed individually about their relationship experiences during home interviews each year for three consecutive years. Analyses focused on two relationship dimensions: emotional intimacy and control. Adolescents reported more intimacy with friends than with siblings and more control with siblings than with friends. In the face of these overall patterns, developmental changes in both firstborns’ and secondborns’ reports of intimacy differed across relationships, with sibling intimacy increasing and friendship intimacy decreasing; declines were evident in these two relationships in the case of control. Associations between sibling and friendship experiences were more apparent for control than for intimacy and for secondborns as compared to firstborns. Discussion focuses on the conditions underlying connections between adolescents’ interpersonal relationship experiences.