Empirical evidence shows that sibling structure influences children’s educational outcomes: While the negative effect of sibling number is fairly consistent, there are mixed findings for birth order and birth spacing. According to the resource dilution hypothesis, differences between siblings arise because siblings must share family resources. Having a greater number of siblings, being a later child, and narrow age gaps between siblings can affect the parental resources available to each child, which in turn can negatively affect educational outcome. To examine the effects of sibling structure, we focused on children’s vocabulary proficiency using longitudinal data from the BiKS-8-14 study at the end of primary school. The results show an expected negative effect for the increased number of siblings, especially if the children come from families with lower levels of education. Regarding birth order, there are also different effects from parents’ education, as only children from less educated families suffer from being a later born child.