It is known that heterozygosity affects mate selection in various taxa. But also beyond mating, in species with parental care, heterozygosity can modulate interactions among family members and their fitness-related decisions. However, to date, this question remains little explored. Here, we studied whether nestling heterozygosity was related to parent-offspring interactions and sib-sib competition in the blue tit, Cyanistes caeruleus, while controlling for the degree of relatedness among nestlings. It has been argued that heterozygosity-fitness correlations are more easily detected under demanding environmental conditions. Thus, we also investigated whether the decision rules of family members according to offspring heterozygosity were affected by brood size, as a proxy of the strength of sibling conflict. First, we found that chick individual heterozygosity was positively although weakly associated with individual body mass. Mean brood heterozygosity did not predict fledging success, but broods that fledged more chicks showed a higher number of less common alleles. Interestingly, fathers, but not mothers, favored heterozygous broods with many nestlings, that is, heterozygous broods with higher potential for sibling conflict. Moreover, the lower the mean brood heterozygosity the stronger the begging intensity when parents were absent, regardless of brood size. Finally, the degree of relatedness among nestlings was not associated with any behavioral parameter, supporting a more prominent role for heterozygosity in shaping intra-family interactions. Importantly, our findings suggest sex-specific rules of parental care according to offspring heterozygosity and that genetic diversity is associated with lower sibling competition.