BackgroundAs the life expectancy of people with intellectual disability increases, they may now outlive their parents or their parents' ability to continue to care. Siblings of adults with intellectual disability often succeed their parents as primary carers. Little is known about the health and well‐being of this important cohort of carers who will be both caring for and ageing alongside their brother or sister with intellectual disability. The Carer's Study within The Intellectual Disability Supplement to the Irish Longitudinal Study on Ageing (IDS‐TILDA) provides a unique insight into the health and well‐being of family carers of older adults with intellectual disability including sibling carers in Ireland. This article reports on the findings from waves 1, 2, 3 and 4 of the IDS‐TILDA Carer's Study.MethodsThe IDS‐TILDA Carer's Study surveys family carers of older people (aged 40 years and older) with intellectual disability on a 3‐yearly cycle. The family carers complete a self‐administered, mixed‐methods questionnaire. The qualitative data are analysed thematically, and a descriptive analysis of the quantitative data is conducted using SPSS.FindingsIn each wave, sibling carers comprised a significant proportion of the Carer's Study participants: W1 58%; W2: 61.4%; W3: 76.7%; and W4: 45.8%. The siblings were predominantly female, not in paid employment and were caring without the support of a spouse or partner. Across the four waves, siblings reported good general health. However, a high prevalence of particular conditions including back pain, aching joints and stress was also reported, and in each wave, siblings reported feeling completely overwhelmed by their care responsibilities. Although sibling carers reported that they have considered the future, only a minority have progressed to action in this regard.ConclusionsThere is evidence of the strong relational and emotional commitment by the siblings to their brother or sister with intellectual disability. However, through the four waves of data, there was also evidence of physical, mental and financial toll. An urgent need exists to identify and engage with siblings to protect their well‐being as they provide care for an increasingly ageing sibling.