This RCT investigated whether participants' sibling configuration moderated the effect of a Theory of Mind (ToM) intervention for children with autism. Children with autism aged 8-13 years (n = 141) were randomized over a waitlist control or treatment condition. Both having more siblings, as well as having an older sibling were related to better outcomes on measures of ToM-related behavior and social cognition, but not ToM knowledge or autistic features in general. The finding that these associations were limited to "practical" skills addressed in the intervention, seems to indicate that having more siblings and having an older sibling provides enhanced opportunities for children with autism to practice taught skills in the home environment.