Using data from administrative registers for the period 1970-2007 in Norway and Sweden, we investigate the intergenerational transmission of multipartner fertility. We find that men and women with half-siblings are more likely to have children with more than one partner. The differences are greater for those with younger versus older half-siblings, consistent with the additional influence of parental separation that may not arise when one has only older half-siblings. The additional risk for those with both older and younger half-siblings suggests that complexity in childhood family relationships also contributes to multipartner fertility. Only a small part of the intergenerational association is accounted for by education in the first and second generations. The association is to some extent gendered. Half-siblings are associated with a greater risk of women having children with a new partner in comparison with men. In particular, maternal half-siblings are more strongly associated with multipartner fertility than paternal half-siblings only for women.