In the early 1960s, WD Hamilton, arguably the most distinguished Darwinian since RA Fisher, formalised an idea that had been knocking around since Fisher and Haldane. If a gene happens to arise which works for the benefit of a sibling, say, or a niece, that gene can survive in the same kind of way as a gene that works for the benefit of offspring or grandchildren. A gene for sibling care, under the right conditions, has the same chance of surviving in the gene pool as a gene for parental care. A COPY IS A COPY IS A COPY, whether it sits in a lineal or a collateral relative.
“Inclusive fitness” was coined as a mathematical device to allow us to keep treating the individual organism (“vehicle”) as the LEVEL OF AGENCY, when we could equivalently have switched to the gene (“replicator”). You can say that natural selection maximises individual inclusive fitness, or that it maximises gene survival. The two are equivalent, by definition. On the face of it, gene survival is simpler to deal with, so why bother with individual inclusive fitness? Because the organism has the appearance of a purpose-driven agent in a way that the gene does not.
Alain Delon & Monica Vitti, L’eclisse, Michelangelo Antonioni, 1962.