This paper studies how sibling gender composition affects preferences for education within Science, Technology, Engineering, and Mathematics (STEM). To identify the causal effect of sibling gender, I focus on a sample of firstborn children who all have a younger biological sibling. The randomness of the younger siblings' gender allows me to estimate the causal effect of having an opposite compared to same sex sibling. Overall, having an opposite sex sibling makes educational choices more gender-stereotypical for both genders. Having an opposite sex sibling reduces women's probability to enroll in any STEM program after compulsory schooling by two percent and to complete a STEM college major by nine percent. Men, in contrast, show an increased interest for the STEM field but are not more likely to succeed in high-level STEM programs. An important mechanism for these findings is changes in child-parent interactions. Parents with mixed sex children gender-specialize their parenting more and spend more quality time with their same sex child than parents with same sex children. Moreover, I show that young boys with an opposite sex sibling are exposed to more gender-stereotypical behavior within the family than boys with a same sex sibling.