Using social control theory and attachment theory as guides, this study examined how qualities of young adolescents' social relationships (i.e., mother, sibling, and friend) and dynamic interactions among characteristics of those relationships are associated with school involvement and delinquency. The participants included older siblings (M age = 14.3), younger siblings (M age = 11.6), and their mothers from 434 families who completed web-based surveys. Results were largely consistent with tenets of social control theory and attachment theory. Young adolescents' social relationships mostly worked in additive ways, but sometimes in compensatory ways for older siblings, to promote positive adjustment. The results suggest that young adolescents' social relationships may be differently associated with adjustment depending on birth order or developmental stage.