The present study contrasted the later life sibling relationships, patterns of family formation, and psychological distress and well-being of siblings of adults with disabilities to a nondisabled normative group. The authors identified 268 siblings of adults with mild intellectual deficits (ID) and 83 siblings of adults with mental illness (MI) from the Wisconsin Longitudinal Study (R. M. Hauser & W. H. Sewell, 1985; R. M. Hauser, J. Sheridan, & J. R. Warren, 1998), a prospective longitudinal study that followed participants from age 18 years to age 64 years. Compared with the norm (n = 791), siblings of adults with mild ID had more contact with family members and were more likely to live in the same state as the sibling with the disability but reported less affective closeness. Siblings of adults with MI reported more psychological distress, less psychological well-being, and less adaptive personality characteristics compared with the norm, particularly for siblings of men with MI. There were no differences between groups in the patterns of marriage and childbearing.