Women are less likely than men to run as candidates in political elections. One reason for this is gendered upbringing, which depresses political ambition among women and strengthens such ambition among men. Furthermore, gendered upbringing can be more pronounced when parents have children of both sexes, and political ambition is often associated with masculine traits. Based on these previous findings, we therefore test the theory that both women and men have a higher likelihood of becoming a political candidate if they have sisters rather than brothers. To establish whether the likelihood of running for office is affected by sibling sex composition, we utilize the fact that nature randomly assigns the sex of the younger sibling when parents decide to have a second child. Using data covering the entire adult Danish population and every candidate in national and local elections between 1990 and 2015, we find, however, no evidence that men and women with a younger sister are more likely to run for office. These findings run counter to previous findings on the effects of siblings and gendered upbringing.