In rats, mating during postpartum estrus and late weaning of the previous litter lead to the overlapping of two litters of different ages within the maternal nest, creating a complex early-life environment for the pups. This reproductive condition also presents a challenge to mothers, who flexibly adapt their behavior to meet the distinct characteristic and needs of both litters. This behavioral flexibility is further reflected in greater cognitive flexibility, measured by the Attentional Set Shifting task. Additionally, in this complex family structure, juveniles exhibit caregiving behaviors toward their younger siblings, shaping different developmental trajectories. Pups from the junior litter show reduced anxiety-like responses in males and females, as well as diminished endocrine stress responses and reduced reproductive behavior in females during adulthood. Furthermore, female offspring from overlapping litters display decreased licking behavior of pups, increased time spent off the nest, and changes in nursing postures when they become mothers themselves. These long-term effects could be attributed to the more complex and enriched rearing environment compared to that of single litters, likely resulting in altered quality and quantity of stimulation from both mothers and siblings. These findings highlight the importance of family structure and early social experiences in modulating maternal behavior, stress responsiveness, and reproductive strategies across generations, and validate the Overlapping litters Model as a valuable framework to study how early-life social and environmental complexity shapes developmental trajectories.