Half your siblings are born with the gene, but they all promptly commit suicide and only the ones without it live to get the benefit. In general, altruistic acts that involve a risk of death make the relatedness value misleading. This does not invalidate inclusive fitness overall, but Hamilton's rule becomes more complex. Where an altruistic act involves a risk of death, you have to either discount the relatedness or benefits to account for the losses among carriers of the gene. This is a special case of a situation mathematically described by saying costs and benefits are multiplicative rather than additive.
Relatedness indicates similarity beyond the average for the species.
Hamilton's rule can't work because animals can't be performing complicated pedigree calculations all the time. The rule states conditions under which the gene will be favored by selection, not a calculation the animal has to perform. The animal does not have to know its behavior is adaptive, or why it is adaptive, for selection to work. The rule does not require extremely complex behavioral rules that divide up all resources proportionate to relatedness. You have some way of specifically identifying your relatives from among all the individuals you meet.