Mate choice and multiple paternity have been widely studied in natural populations, especially in research assessing inbreeding avoidance mechanisms. Ecological factors are expected to affect the costs and benefits of mate choice and multiple paternity, for instance, through their effects on the availability of partners. However, the relative importance and variation of those costs/benefits across fluctuating environmental contexts remains to be established. Here, we used reproduction data collected over 18 years on a wild population of Eastern chipmunks (Tamias striatus) to assess the influence of relatedness among mating partners on their reproductive success and on their offspring fitness in different breeding contexts. In southern Québec, chipmunks live in a pulse resource system where they anticipate masting events of the American beech (Fagus grandifolia) and breed during the summer preceding and/or the spring following a mast. We found that, within a litter, less genetically related sires were assigned more offspring than more closely related ones. This relationship was significant during the summer breeding seasons only, which is characterized by high availability of food and mating partners in the environment. Multiple paternity was also more frequent during summer breeding than during spring breeding. We found no additional effect of parental relatedness on the juvenile survival, longevity, or reproductive success of their offspring. Our results could suggest the presence of context-specific inbreeding avoidance mechanisms by females or differential mortality of offspring at early stages linked to inbreeding depression. Altogether, our findings provide a better understanding of the influence of fluctuating environments on reproduction in small mammals.