This study investigates the direct and indirect effects of maternal and sibling relational intimacy on adolescents' volunteering behaviors via their social responsibility values. Participants included two adolescents (50% female; M age = 1 year) and one parent (85% female; M age = 45 years) from 682 families (N = 2046) from an ongoing longitudinal study. Adolescents self-reported their intimacy with mothers and siblings (Time 1), social responsibility values (Time 1), and volunteering (Times 1 and 2); parents reported on sociodemographic characteristics (i.e., gender, birth order, and family income). Results from a structural equation model indicated that after accounting for adolescents' earlier volunteering, both maternal and sibling intimacy were indirectly related to greater volunteering via social responsibility values. There were no significant direct effects from maternal or sibling intimacy to adolescents' volunteering. Results indicate that both mothers and siblings are important in socializing prosocial and civic values and behaviors during adolescence.