While previous research in general observes adverse effects of siblings on children, less is known about the ways in which material circumstances condition these effects. Using propensity score matching on longitudinal data, this study estimates the effects of being an only child, being born first, and having a large sibling group on adult earnings. Estimates are made for poor and wealthy children respectively in order to examine whether effects occur for both groups. The results show that being an only child, or having a large sibling group, impacts negatively on adult earnings among poor children. Having one younger sibling furthermore has a positive effect on adult earnings among poor children. No corresponding effects were observed for wealthy children. The results indicate that sibling effects are not linear and that they are dependent on family resources