In this study, we examine instrumental and affective involvement in the sibling relationship for adults who have a brother or sister with an autism spectrum disorder (ASD) or Down syndrome (DS). We ask three research questions: (1) How do adult siblings of individuals with ASD differ from siblings of individuals with DS in their assessment of the quality of the sibling relationship and their experience of growing up with a brother or sister with a disability? (2) Are there gender effects on the sibling relationship and sibling experience in these two groups? (3) Which factors are predictive of variation in the sibling relationship for siblings of adults with ASD or DS? Data from 154 siblings who participated in two linked longitudinal studies were used. Seventy-seven siblings with a brother or sister with ASD were matched by age and gender to 77 siblings with a brother or sister with DS. The siblings in each group were between 21 and 56 years of age and over half were sisters. Siblings completed questionnaires on instrumental and affective involvement with their brother or sister with ASD or DS, the impact of growing up with a brother or sister with a disability on their lives, and their coping skills and feelings of pessimism. Compared with the siblings of adults with DS, siblings of adults with ASD had less contact with their brother or sister, reported lower levels of positive affect in the relationship, felt more pessimistic about their brother or sister's future, and were more likely to report that their relationships with their parents had been affected. For siblings of adults with ASD, a closer sibling relationship was observed when the sibling had lower educational levels, lived closer to the brother or sister with ASD, used more problem-focused coping strategies, and when his or her brother or sister with ASD had higher levels of functional independence. In contrast, for siblings of adults with DS, a closer sibling relationship was observed when the sibling did not have children, had lower levels of education, lived closer to the brother or sister with DS, when he or she used more problem-focused coping, was less pessimistic about the brother or sister's future, and when his or her life had been impacted to a greater extent by growing up with a brother or sister with DS. We discuss the implications of these findings for future caregiving roles for siblings. Siblings of individuals with ASD may face difficulty when their parents are no longer able to be the primary caregivers for their brother or sister with ASD, as they tend to have less emotional closeness and are more pessimistic about their brother or sister's future than siblings of individuals with DS. Moreover, in both groups, a closer sibling relationship was observed when the sibling used more problem-focused coping strategies, which may have implications for intervention.