In order to understand the interdependency between intergenerational and adult sibling relationships, a family systems perspective is applied to identify a smaller—empirically analyzable—relational unit of analysis, that is, the sibling–parent–sibling triad. Using balance theory, triadic configurations are derived that represent enhancement, compensation, and loyalty conflicts. The hypotheses are tested for three relational dimensions: support exchange, contact, and conflict. Multilevel analyses of 549 sibling–parent–sibling triads from the Netherlands Kinship Panel data show strong evidence for enhancement, whereas some indication was obtained for sibling relationships being affected by loyalty conflicts. The results underscore and substantiate interdependency between intergenerational and adult sibling relationships.