We share half our genes with our siblings and children; one-quarter of our genes with our grandchildren, nephews, and nieces; one-eight with our cousins and our nephew and nieces’ children.
There is a competition between “inclusive” and “personal” fitness. In species that live with close kin, we observe surprisingly altruistic behaviour, such as squirrels who squeal loudly to warn others of a predator. In the process, alerting the predator to their presence and likely serving as a sacrifice. There is research that shows that humans are skilled at estimating kinship based on faces.