Comprehensive improvements in medical care, technology and residential settings have resulted in persons with developmental disabilities (DD) advancing to older age and outliving parental caregivers (Heller & Arnold, 2010). Typical siblings are expected to become the primary caregiver to their sibling with DD when parents become ill or die and unable to provide care (Burke, Fish, & Lawton, 2015; Heller & Arnold, 2010). This dissertation looks at the wellbeing and family functioning of siblings who become the co-residential caregiver following the transition of a brother or sister with DD from parental to sibling co-residential care. The family systems framework was the theoretical lens for understanding caregiver wellbeing and overall family functioning. Hermeneutic-narrative inquiry was the approach for interviewing and exploring the stories of 10 sibling caregivers of a brother or sister with DD following their transition from parental to sibling co-residential care. Two analytical approaches were used. Firstly, structural analysis involved a within-case analysis of individual participants' stories of transition to determine the meaning ascribed to and identified with the caregiving experience. Secondly, thematic narrative analysis included an across-case analysis to identify themes related to caregiver wellbeing, family functioning, reciprocity of mutual support, and anticipating the caregiver role versus actual experience. Findings from the structural analysis showed that the meaning of the caregiving experience included a duty, obligation, responsibility and commitment to the family. Results from the thematic narrative analysis showed overall lower social and emotional wellbeing among participants, reduced functionality among family members with respect to lower emotional and social functioning, reduced engagement in recreation/leisure activities, as well as lower economic functionality for sibling caregivers with no spouse or children. Sibling caregivers reported higher overall wellbeing and family functioning due to availability of formal supports (e.g., respite care, day program services), and informal support, such as having support from a spouse, child, or extended family member. Findings regarding reciprocity showed increased instrumental support among sibling caregivers and reduced emotional support. When anticipating the role, caregivers described knowing they would assume the role but were unclear of the shift to assuming a parental rather than sibling role. Other unanticipated discoveries included feeling captive to the role and feelings of helplessness. Caregivers' actual experiences involved learning to manage new challenges, society's patronizing view of persons with DD, and an overall sense of pride in caregiving for giving back to their sibling with DD. Grief and future planning were also discussed, including the effect of grief on the sibling caregiver, sibling caregiver's children and sibling with DD. Future planning looked at the aspects of planning and not having planned for the future of the sibling with DD. The study concludes with implications for current and future social work practice and research, as well as the study's strengths and limitations.