The goal of the present study is to examine differences in cognitive, psychomotor, and social-emotional development scores between only children and children with siblings aged 6-30 months in rural China using data of 1802 children across 351 villages in 174 townships, disaggregating according to child gender. The results indicate that having a sibling is negatively associated with cognitive, psychomotor, and socioemotional development for girls, but not for boys. Despite this, girls appear to be developmentally advantaged compared to boys, even when they have a sibling. Our results, however, are not conclusive regarding why only-girls are affected while boys are not affected by the existence of siblings. Parenting behaviours (such as playing with children, reading to children, reading to children with books, and singing to children), appear to be unrelated to the developmental differences between only-girls and girls with siblings.