In this project, we analyze the causal effect of sibling gender on attitudes and preferences. Comparing first-born women with a next-born sister to first-born women with a next-born brother allows us to estimate the causal effect of sibling gender. In particular, we find that a next-born sister leads first-born women to have less stereo-typically female preferences in education and in terms of importance of career. Furthermore, we find that women with a sister have higher life satisfaction and less mental health issues. We also explore how the gender of the next-born sibling influences family relationships, in order to say more about the mechanisms at work. Our findings high light, that the relationship with the sibling changes with sibling gender while those with the parents remain constant. In addition we find that women with a sister become more similar to men with respect to their social skills and views on gender norms. These results shed light on how sibling gender influences preferences and attitudes, specifically those for education choices that are gender role conforming.