In winter southern flying squirrels (Glaucomys volans) aggregate in large groups. They live on food items individually stored during the fall in their overlapping home ranges. The squirrels gain thermoregulatory benefits from living in aggregations but also face costs of group living, especially nest mates pilfering individually stored food. Other costs include increased predator attraction and a greater vulnerability to parasite infection. The presence of relatives in the group has the potential to increase inclusive fitness by increasing the availability of food, stored in the home area, to related individuals. Using 3 generations of known-relationship squirrels we conducted laboratory experiments to determine whether kin or familiar animals were preferred nest mates during aggregation formation. During 3 time periods, over 2 winters, squirrels were presented with kin and nonkin and familiar and unfamiliar animals and allowed to aggregate over the course of multiple 3-day trials. Kinship was persistently a major factor in the formation of aggregations. Squirrels aggregated with highly related animals (parents, offspring, and siblings) significantly more often than with unrelated animals. Familiarity became significant by the end of the study. Understanding how relatedness and familiarity interact in the formation of aggregations in seasonally gregarious animals sheds light on the processes and factors that lead to sociality.