I define a necessary resource as something that in sufficient quantity virtually ensures (for the time being; everything dies) the organism’s survival. Something like that.
Incidentally, I’m not thinking only about humans here, though I understand the urge to focus on the species we know best. The original question might be rephrased more specifically, as “Why does a hatchling, who is ~50% genetically identical to its siblings, toss a sibling out of the nest while its parent, who is ~50% genetically identical to its offspring, feeds it?” Or “Why does a child, who would be ~50% genetically identical to a potential sibling, try to prevent the conception of said sibling, while the parents, also ~50% genetically identical to a potential offspring, try to bring about the conception of said offspring?”
So you’re looking for a biological evolutionary reason then. I think some answers were provided in that regard. Notable is the difference in behavior between baby birds and humans.
And post #2 comprehensively answered that narrow question.
The difference is the genes in the parent “know” they reproduce themselves via convincing their host parent to make & raise baby. The genes in the sib “know” the sib ain’t them and doesn’t have their genes and is of no use to them. And in fact is of negative use to them.
Genes are not smart. They are however ruthlessly selfish. They were originally “designed” more for K-style reproduction and actually have some difficulty playing nice with R-style reproduction. As was explained in @Chronos’ post #12.
The young William Marshal had been given to King Stephen of England during the Anarchy as a hostage against the good behavior of his father John. Said good behavior not being forthcoming, Stephen threatened to hang the six-year old William in full view of a castle his father was defiantly holding against Stephen. To which John Marshal replied, paraphrased, to go ahead - he could always make more sons 
.
Of course, he already had several sons and had correctly gauged Stephen’s character, so nothing came of it. But basically the human capacity to reason tends to stand natural selection on its head.
There’s also the matter of each sibling learning from their competition with each other. Eventually they’re going to grow up and compete with people that aren’t related. They’ll both be better off having “practiced” in a safe setting so they can be more prepared for the wider world.