Sibling relationships are usually lifelong and reciprocal. They can assume particular significance when a brother or sister has a learning disability. Until recently, adult siblings of people with disabilities such as severe autism have been ignored by policy, practice and research. This qualitative study contributes to an emerging literature by exploring how adult siblings, who have a brother or sister with autism (plus learning disability) and living in England, give meaning to their family (and caring) relationships and engage with service delivery. We spoke to 21 adult siblings using semi-structured interviews and met with 12 of their siblings with autism. Our analysis, using a broad narrative approach, demonstrates the continuity of the sibling relationship and an enduring personalised commitment. The nature of this relationship, however, is sensitive to context. How non-disabled adult siblings relate to their childhood experience is fundamental when making sense of this, as is their need to fulfil other social and family obligations, alongside their 'sense of duty' to support their disabled brother or sister. Sibling experience was further mediated by negotiating their 'perceived invisibility' in social care policy and practice. Our work concludes that by understanding the way relationships between siblings have developed over time, adult siblings' contribution to the lives of their brother or sister with autism can be better supported for the benefit of both parties. Such an approach would support current policy developments.