Group living is widespread in animals. In nature, groups usually not only differ in phenotypic characteristics but also in the social relationships among group members. Theory predicts that individuals adjusting their shoaling decisions—to join certain groups or not—based on social criteria, such as familiarity or genetic relatedness, can increase their fitness. Although numerous studies report grouping preferences based on social criteria, the benefits actually emerging from such behavioral preferences are less well studied. Here, we examine both shoaling preferences and their consequences in juveniles of Pelvicachromis taeniatus, a monogamous cichlid fish from Western Africa with biparental brood care. After juvenile P. taeniatus have left their parents, they form loose shoals. Then, juveniles may have the option either to stay in their sibling group or to join a novel shoal. Therefore, in a first experiment, we tested whether juveniles prefer to shoal with their familiar sibling group or a group consisting of unfamiliar unrelated individuals. Second, we examined whether the shoaling decision translates into fitness benefits. We used body size as proxy for fitness and compared the growth in groups consisting exclusively of familiar full-siblings with growth in groups consisting of members of different relatedness and familiarity. Juvenile P. taeniatus preferred shoaling with kin over shoaling with non-kin. Growth was significantly higher in kin-only groups than in mixed groups indicating that grouping with familiar kin yields fitness benefits in juvenile P. taeniatus. Our results suggest that individual shoaling decisions based on social criteria can be adaptive.