Empirical evidence suggests that sibling structure influences children’s educational outcomes: While the negative effect of the number of siblings is quite consistent, there are mixed findings for birth order and birth spacing. According to the resource dilution hypothesis, differences between siblings occur because siblings have to share family resources. Having a larger number of siblings, being a later-born child as well as narrow age gaps between siblings can affect the parental resources available for each child, which may thus negatively affect educational outcome. To study the effects of sibling structure, we use longitudinal data from the BiKS-8-14 study at the end of elementary school, focusing on children’s vocabulary competences. Our results indicate an expected negative effect for increasing number of siblings particularly when children originate from families with a lower educational background. Regarding birth order, we also find differential effects by parents’ education, as only children from less educated families suffer from being a later-born child. No effects can be identified for children’s birth gaps in relation to younger siblings as soon as number of siblings is being considered, whereas longer spacing between a child and his/her older siblings is positively related to vocabulary competences. With respect to possible changes across time, sibling effects appear to be rather stable at the end of primary education.