Three empirical studies comprising this dissertation were carried out to provide a more comprehensive understanding of concurrent measures of both predictors and outcomes of young adolescents' sibling interactions. The participants included older siblings (M age = 14.3), younger siblings (M age = 11.6), and their mothers from approximately 450 families who completed web-based surveys.Study one examined correlates of overt and relational aggression between young adolescent siblings. Family environment emerged as an important factor in explaining internalizing problems as well as overt/relational aggression for both younger and older siblings. Findings also provided partial support for the positive linkages between young adolescents' aggression and their own and siblings' internalizing problems above and beyond family and maternal variables. The main purpose of the study two was to investigate self-criticism as a potential mediating factor in the link between mother-child relationships and aggression or perceptions of social competence. Maternal self-criticism strongly corresponded to the quality of relationships with children, and an intergenerational similarity of self-criticism, particularly for older children, was found. Mother-child relationships and adolescent self-criticism were significantly related to aggression and perceptions of social competence. In addition, the link between mother-child relationships and aggression or social competence is mediated, in part, by self-criticism in adolescents. Using social control theory and attachment theory as guides, study three 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. Results were largely consistent with tenets of social control theory and attachment theory. Young adolescents' social relationships were associated with their adjustment in ways that are protective or beneficial. Furthermore, these social affordances worked additively and in compensatory ways to promote positive adjustment. Positive relationship qualities tended to amplify other relational influences promoting successful development, and these findings were invariant across gender.While optimal mother-child relationships are central in nurturing the child's capacity to develop warm and harmonious sibling relationships in both a direct and an indirect manner, there is a considerable contribution of siblings to adjustment and socialization above and beyond the effects of mother-child relationships.