Behavioural adjustment of children with intellectual disability and their sibling is associated with their sibling relationship quality

Background: Understanding sibling relationship quality is important, as it is associated with mental health outcomes in both childhood and adulthood. Arguably, these relationships are even more important for individuals with intellectual disability, as siblings can be important sources of care, support, advocacy and friendship for one another. The intellectual disability field, however, has a tendency to assume that the relationship lacks reciprocity, and that it is the sibling with intellectual disability who affects the sibling, and that this effect is somehow ‘negative’. Methods: We examined whether the behaviour problems and prosocial behaviour of 500 child sibling pairs, where one child has an intellectual disability, were associated with their sibling relationship quality. Measures included the Strengths and Difficulties Questionnaires and the Sibling Relationship Questionnaire. Family poverty, the gender of both children, birth order and whether the child with intellectual disability had autism or Down syndrome were also included in the analyses. Results: Confirmatory factor analysis indicated an adequate model fit for the latent variables measuring sibling relationships. The final structural model found that the prosocial behaviour and internalising problems of the children with intellectual disability, their typically developing siblings' prosocial behaviours and sibling birth order were associated with intimacy–companionship in the sibling relationship. The internalising, externalising and prosocial behaviours of the children with intellectual disability, their siblings' externalising behaviours and sibling birth order were associated with antagonism–quarrelling in the sibling relationship. Conclusions: We found that the behaviours of both the child with intellectual disability and their sibling were associated with both ‘positive’ and ‘negative’ dimensions of their sibling relationship. This indicates a bidirectional and reciprocal effect