Family care provision is the norm for adults with intellectual and developmental disabilities (I/DD), even as they and their support networks grow older. As families age together, the role of primary carer frequently transitions from the parent to a sibling, as aging parents die or become too frail to provide continued support. The purpose of this paper is to explore the transition in care from the perspective of a sibling who has replaced parents as the primary carer for an individual aging with I/DD. Data are drawn from semi-structured, in-depth interviews with a sample of adults over age 40, living in the United States, and caring for a sibling with I/DD (n = 15). Data were analyzed using a constant comparative qualitative approach. Results reveal themes impacting the adjustment to the role of primary carer, the extent to which aging transformed the content of care needs, the importance of planning, and the availability of supplementary support. Findings from this study underscore the need to develop long-term services and supports as well as educational resources that accommodate this population of carers as they age together with their sibling with I/DD.