Is anyone familiar with Carol Gilligan's works, and her writings on the ethics of care? She argues that women have a different orientation toward ethics than men do, in terms of what is fair.
It is ethical to favor your own child versus the children of strangers. Isn't the love and compassion you fell toward your own child naturally stronger that that you feel toward a stranger? Extend that to more distant family members. Perhaps the favoritism diminishes, but it is still stronger than that toward complete strangers.
The nursing culture is largely female. If Gilligan's ethical theories have any currency, than they go some way toward explaining certain aspects of the nursing culture.
Which other organizations are dominated by females? Female subcultures in secondary schools? College sororities? The only other on I can think of is Catholic nuns. I'm not sure that any of those make for a valid comparison.