Think about it in terms of particular mutations causing altruistic behavior, and the conditions under which they spread. Also, remember that when a mutation that causes altruistic behavior towards some group first arises, it is rare.
Say that I spontaneously mutated in conception to be altruistic towards siblings (via the mechanism of liking agemates I grow up with in sufficiently close contact). Then half of my children carry the dominant mutation and help each other, so that even though the mutants help non-mutant siblings, a large enough fraction of aid goes to other mutant siblings that the gene becomes more frequent in the population.
On the other hand, imagine a new mutation causes altruism towards some broad group, like tall people. My mutant children will spend most of the resulting altruism on tall strangers who lack the gene.
This sounds correct, but of course we also know both with humans and other animals that there are some net advantages to various herding and familial instincts even if those don't by necessity favor those most closely related (although on average, the locals are more closely related to you than those not in your tribe/troop/herd/pride etc.). There is of course competition both within and between species; I think this is all that the original post is hung up on.
The standard story when dealing with inclusive fitness in evolutionary arguments is that my sibling's life is worth half of mine and my cousin's life is worth a quarter of mine.
But I obviously share more than half my genes with my sister because my parents are not unrelated. My parents must share a lot of ancestors enough generations back that nobody has tracked them, since they resemble each other insofar as they both look human. If I take into account that my parents are both human, I should be related to my sister much more than 50%.
So why do they assume a sibling has half your genes when reasoning about inclusive fitness?
My wife is Chinese, and both of my parents are of European descent. Should I expect my kids to like each other less than I like my sister, because they are less closely related?
This is an intellectual question about evolutionary psychology, not an anxious question about my family relationships. We're all doing fine, don't worry.