This study examines the hypothesis that having an older sister causes one to perform relatively better at reading. For the analysis, a cross-subject analysis is conducted to examine a student's relative reading test score (reading test score minus math test score) based on older sibling gender. We found that a student's relative reading test score is larger when the student has an older sister than when he or she has an older brother. Further analyses show that although conversation frequency does not vary based on older sibling gender, siblings are more likely to talk about studying, career paths, or school life when an older sibling is a sister than when an older sibling is a brother.