To test divergent theoretical predictions as to the impact of having a younger or older, same-sex sibling or opposite-sex sibling on other gender constancy, Israeli kindergarten children in two-child families responded to a gender constancy task in which a male and female picture target engaged in counter-stereotypic toy play and adopted counter-stereotypic appearance. Children were also asked whether the target child could change sex at will and to explain the difference between boys and girls. Relative age of sibling, both independently and in interaction with participant sex, was associated with greater gender constancy in the face of counter-stereotypic toy play. Relative age of sibling and sibling sex was independently associated with greater gender constancy in the face of counter-stereotypic appearance. In both cases, children with a younger sibling evidenced greater gender constancy. Boys, especially ones with a male sibling, referred most explicitly to target's birth sex in accounting for his or her being unable to change sex at will. Genital and anatomical differences between boys and girls were cited more often by children who referred explicitly to target's birth sex and had a younger sibling. The results were discussed in the context of theories of gender development and the socialization of gender within the family. (Contains 5 tables and 6 notes.)