Evidence exists for two competing theories about the effects of having an older sibling on development. Previous research has found that having an older sibling has both advantages and disadvantages for younger siblings' development. This study examined whether and how older siblings influenced the onset of their own younger siblings' motor milestones, a heretofore unstudied developmental domain in the sibling literature. Parents of 51 sibling pairs reported their children's crawling and walking onset dates. In keeping with imitation theories, in families where younger siblings crawled or walked earlier than their own older sibling, they did so significantly earlier. Moreover, in keeping with limited parental resource theories, in families where older siblings crawled or walked earlier than their own younger sibling, they did so significantly earlier. Older siblings did influence younger siblings' motor development, but how they did so may have depended on unique family characteristics.(Contains 1 table.)