Studying the evolution of cooperative breeding and group living requires simultaneous quantification of both helping benefits and competitive costs within groups. Although such research has traditionally focused on the fitness benefits of helping behavior, increasing evidence now highlights reproductive competition in cooperatively breeding animals including humans. Human groups consist of cooperative individuals of varying relatedness, predicted to lead to conflict when resources are limited and relatedness low. However, few studies exist that determine the costs of co-breeding to both parties sharing resources. Here, we studied female reproductive competition in historical Finnish joint-families where brothers stayed on their natal farms and sisters married out, so that several unrelated women of reproductive age co-resided in the same households. Using detailed parish registers we quantified the effects of simultaneous reproduction of these women on their offspring mortality. We found that the risk for offspring mortality before adulthood was increased by 23% if co-resident women reproduced within 2 years of each other, a risk that was not associated with the overall numbers of co-resident reproductive-aged women or children. Such costly competition may have promoted the evolution of birth scheduling, dispersal patterns and life-history traits including menopause that avoid resource competition with other reproductive females.