Background: Given the increasing lifespans of individuals with intellectual and developmental disabilities (IDD), siblings may fulfil multiple caregiving roles simultaneously for their ageing parents, their offspring, and their brother or sister with IDD. Yet, little is known about compound sibling caregivers. The purpose of this study was to compare the perspectives of compound, single and non-caregiving siblings of adults with IDD. Method: This study investigated 332 adult siblings of individuals with IDD in the United States via a national web-based survey. Participants included: 152 non-caregivers, 94 single caregivers (i.e., caregivers only for their brothers and sisters with IDD), and 86 compound caregivers (i.e., caregivers for their brothers and sisters with IDD and at least one other vulnerable individual). Results: Single and compound sibling caregivers (versus non-caregivers) had more positive relationships and conducted greater advocacy and future planning activities. Conclusions: Given the potential for compound sibling caregiving, further investigation is warranted.