It may be perfectly natural for me not to give anything to people in another country (because I don't care about them the way I care about my family), but that natural disparity in my affection for my fellow countrymen and for the foreigners doesn't mean that if we go to war I can commit war crimes against the people of that country.
The objective value imposed by ethics should be operative and overriding when making decisions about who lives and dies, because these choices are not just personal choices that primarily affect the person making the decision. If I ever desperately need medical attention and the doctor in triage has to choose who to treat and who to let die, I hope he has a more meaningful standard than whom he personally happens to like more.
And just to be clear, the criticism shouldn't be taken to say that I think it would have been wrong to perform the brain surgery even at risk to the child's life. It was simply the GGP's glib resolution of the question entirely in terms of the family members' affections (not even of the mother's!), as though those alone were were the only consideration and were of themselves sufficient to justify the conclusion.