Background The potential effects on other children when there is a child with intellectual disability (ID) in the family are being increasingly recognized. This study describes the impact of having a sibling with Down syndrome or Rett syndrome using a questionnaire completed by parents.Methods The parents of 186 Western Australian children with Down syndrome and 141 Australian girls and women with Rett syndrome participated in the study. Patterns of reporting disadvantages and/or benefits were compared across a number of child and family variables (age, functional ability and birth order of the affected child, number of siblings and number of parents in the family home) and by socio-economic status as measured by the index of relative socio-economic disadvantage and by area of residence. Parents' responses to open-ended questions about the benefits and/or disadvantages for siblings of their child were analysed for themes.Results The majority of parents in the Rett syndrome and Down syndrome groups reported both disadvantages and benefits for siblings. In the Rett syndrome group, families from outer regional areas were the least likely to mention disadvantages and those with a smaller family more likely to note disadvantages. In both groups, more socio-economically advantaged families were more likely to report disadvantages. In the Down syndrome group, benefits were also more commonly reported by parents who were socio-economically advantaged, and by larger and two-parent families. Major disadvantages for siblings centred around parental and personal time constraints, relationships and socializing, restrictions, parental emotion and burden of helping. Major benefits were related to personality characteristics.Conclusion Parents identified both benefits and disadvantages to the siblings of their child with either Rett syndrome or Down syndrome. It is important that these findings are incorporated into any discussion around the impact on the family of a child diagnosed with an ID.