Research with siblings of children with Autism Spectrum Disorders (ASD) suggests that they may be at increased risk for behavioural and emotional problems and relatively poor sibling relationships. This study investigated a diathesis-stress model, whereby the presence of Broad Autism Phenotype features in the typically developing siblings might interact with family-environmental risk variables to predict sibling functioning (5-17 years of age) of children with an Autism Spectrum Disorder (ASD), their child with an ASD, and their own psychological well-being. Sibling adjustment was associated with the extent of behaviour problems in the child with an ASD and with the extent of the sibling's Broad Autism Phenotype (BAP) features. Sibling relationships were more negative when the child with an ASD had more behaviour problems and when there was evidence of critical expressed emotion in the family environment. Siblings with more BAP features, who had brothers/sisters with an ASD and a greater number of behaviour problems, had more behaviour problems themselves. Siblings with more BAP features who had parents with mental health problems reported more sibling relationship conflict. (Contains 2 tables and 2 figures.)