An article on our questionnaire study was published in Research in Developmental Disabilities in Nov 2019 (vol 94), in a special ‘virtual issue’ on Sibling Relationships in Developmental Disabilities:

Siblings of children with Williams syndrome: Correlates of psychosocial adjustment and sibling relationship quality
Katie Cebula, Amanda Gillooly, Laura K. Coulthard, Deborah M. Riby, & Richard P. Hastings

Summary:

Children with Williams syndrome can also have intellectual disabilities. They are often very sociable, but may also have anxiety and health difficulties. How does this affect their siblings? To find out, we gave 41 parents/caregivers some questionnaires about the behaviour, emotional difficulties and relationship quality of their child with Williams syndrome and the ‘typically developing’ sibling (both 4 – 18 years old). In 31 families the siblings themselves also took part.
We found that siblings of children with Williams syndrome are, overall, similar to the general child population, in terms of behaviour problems and emotional adjustment (including anxiety). We also found that siblings sometimes reported having more difficulties than parents thought they had. However, siblings also said that they had high levels of positive support, particularly from friends and family. There were also differences between families. Behaviour problems and emotional difficulties were more common in siblings when there was conflict in the sibling relationship.
The findings show that having a child with Williams syndrome does not necessarily cause major difficulties for their siblings. However, it also shows that it is important for support services to listen to the siblings themselves about their experiences. Finally, it shows that a ‘one size fits all’ approach to family support is not useful for families with a child with Williams syndrome.