Sibling relationships are argued to offer a rich social context for the development of young children's theory of mind (ToM). There is evidence that the presence of siblings, particularly older siblings, may promote preschoolers' ToM, but it has not been investigated among toddlers. This study evaluated differences in early manifestations of ToM between toddlers in different sibling compositions: (1) without a sibling, (2) with younger siblings only, and (3) with older siblings only. The sample included 227 toddlers who completed 2 tasks assessing their understanding of discrepant desires and visual perspectives. Results indicated that only children and those who had older siblings did not differ from each other on either task. However, on both tasks, toddlers with younger siblings performed significantly more poorly compared with those with older siblings. Moreover, children with younger siblings demonstrated poorer visual-perspective understanding compared with only children. It may be critical to take siblings' birth order into account when investigating their respective effects on each other's ToM, especially in toddlerhood.