Shared genes give relatives shared interests in each other’s evolutionary success, yet differences in patterns of relatedness can create conflicts. In a monogamous relationship, parents are equally related to all their children and also equally related to all their grandchildren. However, their children are more closely related to their own children and take greater interest in them than in their nieces and nephews. Various types of parent-offspring conflict can be explained in terms of such patterns of genetic relatedness. The authors extend this principle to mother-daughter conflict over choice of the daughter’s partner and to competition between sisters by considering how parental influence causes increased competition among same-sex siblings. The authors conclude that females wish family members to choose partners with traits that may provide more direct benefits and potentially improve their fitness, that individuals choose sexier partners for themselves, and that parental influence may theoretically drive sister competition.