Background: Life expectancy for people with Down syndrome is increasing, but older adults with Down syndrome are at greater risk of developing certain pathologies associated with ageing. When ageing parents can no longer look after their child with Down syndrome, their other children are often expected to shoulder this responsibility. Method: We asked 120 adult siblings of a person with Down syndrome to complete a questionnaire on their current and anticipated relationships. Results: Most respondents considered these relationships to be positive, but they expressed concerns about the effects of ageing and about having to organise their sibling with Down syndrome's future life. We investigated the impact of factors relating to the respondents and to their sibling with Down syndrome (age, gender, social inclusion, protection regime) on these general tendencies. Conclusion: Siblings need to be given support, for example, "via" discussion groups, to facilitate the transition to becoming caregivers.