This study examined social interaction effects on demographic behavior, asking whether fertility and marital decisions spread in the sibling network. Using panel data from the German SOEP (1984 – 2015; N = 1,735 sibling dyads), we estimated multilevel discrete-time hazard models with random effects at the individual and family level to examine whether siblings’ transitions to marriage and parenthood influence an individual’s chance to get married and to have children. Using a simultaneous equations model, transitions to parenthood and marriage were estimated jointly, allowing for correlations between the unobserved individual characteristics that affect marriage and parenthood. Results show that the probability of becoming a parent increased substantially in the year after a sibling had a child. Similarly, the probability of getting married increased after the first year following a sibling’s marriage. We also find evidence for social interaction effects across different transitions in the process of family formation, as the probability of getting married increased further after a sibling had a child, and especially if this sibling was also married. Overall, these findings are in line with a growing body of literature highlighting the contagiousness of demographic behavior in intimate networks.