The authors examined the relationship between number of siblings and false belief understanding (FBU) in 94 low-income 4-5-year-olds. Previous research with middle-income children has shown a positive association between number of siblings and FBU. However, it is unclear whether having multiple siblings in low-income families is related to better FBU. Language, specifically vocabulary, was examined as a possible mediator between number of siblings and FBU as several researchers have found that language is causally related to FBU. Contrary to research with middle-income preschoolers, the authors found that number of siblings was negatively related to low-income children's FBU. This relationship was mediated by children's vocabulary skill. Suggestions for why the sibling-FB relationship may differ in low- and middle-income samples are offered.