Although socioemotional competencies have been identified as key components of youths' positive development, most studies on empathy are cross-sectional, and research on the role of the family has focused almost exclusively on parental socialization. This study examined the developmental course of empathy from age 7 to 14 and the within-person associations between sibling warmth and conflict and youths' empathy. On three occasions across 2 years, mothers, fathers, and the two eldest siblings from 201 White, working- and middle-class families provided questionnaire data. Multilevel models revealed that, controlling for youths' pubertal status and parental education, girls' empathy increased during the transition to adolescence and then leveled off, but boys' lower levels of empathy remained relatively unchanged. Moreover, controlling for parental responsiveness and marital love, at times when firstborns and second-borns reported more sibling warmth and less sibling conflict than usual, they also reported more empathy than usual. The within-person association between sibling warmth and empathy also became stronger over time. Findings highlight gender differences in empathy development and the unique role of siblings in shaping each other's socioemotional characteristics during adolescence. (Contains 2 tables and 1 figure.)