Research indicates a positive relation between the sibling constellation and theory of mind (ToM) development in typically developing (TD) children. Less is known about this association in children with autism spectrum disorder (ASD). The current study examined the association among the presence and number of siblings, birth order, and false belief (FB) understanding in children with ASD and a TD comparison group. Two FB tasks (change of contents and change of location) and the Peabody Picture Vocabulary Test were administered to 57 children with ASD and 28 TD children during a home visit. One parent of each child reported on demographics and the sibling constellation. Separate hierarchical regressions controlled for age, receptive language ability, and scores on the Social Communication Questionnaire. In children with ASD, no association was observed between presence or number of siblings and ToM. However, the presence of older (but not younger) siblings was found to be positively associated with ToM. Children with ASD who had at least one older sibling performed similarly to the TD group, whereas children with ASD who had no older siblings performed significantly worse than the TD group. These findings indicate an advantage for FB performance in children with ASD who have an older sibling. They may bear on decisions to include older siblings or peers in intervention programs and may also contribute to a more complete understanding of the origins of individual differences in ToM ability in children with ASD.