In this project, we analyze the causal effect of sibling gender on attitudes and preferences. Comparing first-born individuals with a next-born sister to first-born individuals with a next-born brother allows us to estimate the causal effect of sibling gender. First, we find that sibling gender matters only for women: men are not at all affected by the gender of their sibling. For women, 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 in comparison to women with a brother have higher life satisfaction and better mental health. 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 highlight, that the relationship with the next-born sibling changes. In addition, we find that women with a sister become more similar to men with respect to their self described social skills. These results shed light on how sibling gender influences preferences and attitudes, specifically those for education choices that are gender role conforming.