Kin selection theory, the most general expansion of Darwin's natural selection theory, has been verified by female biased investment by female workers in eusocial Hymenopterans where relatedness from the worker to sisters is higher than to brothers because of haplodiploidy. However, a strong test of the theory has proven difficult in diploid social insects because they lack such relatedness asymmetry. Here we show that kin selection can result in sex ratio bias in eusocial diploids. Our model predicts that allocation will be biased toward the sex that contributes more of its genes to the next generation because of sex-asymmetric inbreeding. The prediction matches well with the sex allocation of termites. Some termites have an asexual queen succession (AQS) system where queens produce replacement queens asexually but use sexual reproduction to produce a replacement king, which results in mother-son inbreeding. In these species, the sex allocations were significantly female biased while non-AQS species showed equal allocations. These results suggest that kin selection is an important force in both diplodiploid and haplodiploid organisms. Our findings open broad new avenues to test inclusive fitness theory beyond the well-studied eusocial Hymenoptera.