Background There are few published research studies in which siblings of children with autism spectrum disorder (ASD) provide self‐reports about their own behavioural and emotional problems and their sibling relationships. Reliance on parent reports may lead to incomplete conclusions about the experiences of siblings themselves. Methods Siblings 7–17 years and their mothers from 94 families of children with ASD were recruited. Mothers reported on family demographics, the behavioural and emotional problems of their child with ASD, and on their own symptoms of depression. Siblings reported on their relationship with their brother or sister with ASD, and siblings 11+ years of age also self‐reported on their behavioural and emotional problems. Results Compared with normative British data, siblings reported very slightly elevated levels of behavioural and emotional problems. However, none of the mean differences were statistically significant and all group differences were associated with small or very small effect sizes – the largest being for peer problems (effect size = 0.31). Regression analysis was used to explore family systems relationships, with sibling self‐reports predicted by the behaviour problems scores for the child with ASD and by maternal depression. Maternal depression did not emerge as a predictor of siblings' self‐reported sibling relationships or their behavioural and emotional problems. Higher levels of behaviour problems in the child with ASD predicted decreased warmth/closeness and increased conflict in the sibling relationship. Conclusions These data support the general findings of recent research in that there was little indication of clinically meaningful elevations in behavioural and emotional problems in siblings of children with ASD. Although further research replication is required, there was some indication that sibling relationships may be at risk where the child with ASD has significant behaviour problems.