Associations among sibling relations and the psychosocial and illness-specific adaptation of youths (N = 66) with insulin-dependent diabetes mellitus (IDDM) were examined. The findings suggest that sibling relations, especially sibling conflict, contribute an independent source of variance above and beyond that contributed by demographic characteristics, sibling constellation variables, and important dimensions of family relations in predicting the youths' adaptation. High family-life stress and high sibling status/power contributed unique variance in predicting internalizing behaviors, and male gender and sibling conflict contributed independently to externalizing problems. Sibling conflict also contributed unique variance to the youth's general self-esteem, along with social class and family cohesion, and to their adjustment to IDDM. Data suggest that parent-child dyads and sibling dyads represent interrelated and independent subsystems within the family, and that both subsystems may influence the psychosocial functioning of youths with IDDM.