Highlights: Overall, siblings of children with WS do not show heightened behavioural difficulties or anxiety compared to population norms. Caregivers report heightened emotional difficulties in some siblings of children with WS. Caregivers report fewer adjustment and relationship difficulties in siblings of children with WS than do the siblings themselves. Adjustment problems in siblings of children with WS are associated with increased caregiver depression and more sibling relationship conflict. Higher warmth in the sibling relationship is associated with increased prosocial behaviour in the child with WS. Background: Previous research has examined adjustment in parents of children with Williams syndrome (WS), but little is known about sibling outcomes. Aims To explore sibling adjustment and relationship quality, and their demographic, psychological and behavioural phenotypic correlates from the perspective of caregivers and siblings in families of children with WS. Methods and procedures: Forty-one caregivers of children with WS participated in this questionnaire study on the adjustment and relationship quality of the siblings. In 31 of these families, self-report data were also provided by the siblings themselves. Data were also gathered on potential correlates, including anxiety and social functioning in the child with WS, caregiver mental health, and sibling social support. Outcomes and results: Sibling adjustment was similar to population norms, though significantly increased caregiver-reported emotional difficulties were found. Siblings reported greater behavioural, emotional and relationship difficulties than caregivers perceived them to have. Some significant associations were found between the behaviour of the child with WS, sibling behaviour problems and sibling relationship quality. Conclusions and implications: A picture of relatively positive sibling adjustment and relationships emerged, but findings of individual differences and some emotional difficulties emphasise the need for an individualised approach to support in families of children with WS.