While adult children with siblings can share caring for older parents, adult only children face this responsibility alone. Yet, despite the extensive literature on informal caregiving more generally, research on only children’s parent-care is limited. Given increased longevity and reliance on informal caregiving, as well as an increase in one-child families, there is a need to further investigate only children’s caregiving. This paper investigates whether and how adult only children’s parent-care differs from those with siblings, how sibling composition intersects with gender and how it relates to wellbeing. Using data from three large scale British birth cohorts we analyse parent-care at different ages: 38 and 42 (born 1970), 50 and 55 (born 1958), and 63 (born 1946). Results show that only children are more likely to provide parent-care, with differences greater at later ages. Provision is gendered, and the sibling group composition matters for involvement. While caring is related to wellbeing, we found no evidence that this differs between only children and those with siblings.