Research Findings: This study extends the understanding of the sibling effect on children's theory of mind (ToM) among Chinese preschoolers by adopting an ecological perspective. The participants were 225 Chinese preschoolers, comprising 100 children with siblings (M[subscript age] = 4.54 years, SD = 1.11, 55 boys) and 125 children without siblings (M[subscript age] = 4.52 years, SD = 0.99, 60 boys). We assessed children's ToM through a child measurement task, and peer interaction time on the weekend and parenting style (i.e. authoritarian and authoritative parenting style) through parent reports. This study revealed that the role of siblings in children's development of ToM did not have a main effect among Chinese preschoolers, regardless of their birth order or siblings' ages. However, when considering the influence of parents and peers in addition to siblings, the sibling effect is only noticeable for children with low interaction levels with their peers. Practice or Policy: In conclusion, this study demonstrates that in China, child-aged siblings positively influence the ToM development in children who have limited interactions with peers. These findings underscore the value of siblings as a developmental resource and offer parents critical insights into fostering ToM, especially under the two-child policy.