Here's my first stab:
The ancestral environment had little if any interracial couples producing kids. (The old rape-the-neighboring-tribe's women thing still didn't involve vastly different races on the scale of chinese/causasian.) Therefore, kin selection pressures did not perceptably benefit from distinguishing between "siblings from interracial parents" vs. "siblings from more closely-related parents" -- all parents were the latter type.
So it doesn't seem like there would be room for a more fine-grained relative-gene-similarity detector to develop: most of the benefit from helping your kin in the ancestral environment is from the fact of that kinship, and further deviations from the simple percentage-genes-shared calculations (due to the relative relatedness of parents you mention) would just be very weak noise.
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.