I suspect life and death scenarios make for bad moral law - why not consider something more mundane? Say, a birthday cake? How would you share it out amongst your kids?
Well, not winner takes all, I agree.

Hard cases make bad law may be as true of morality as it is of the law, I supopose, although if equality is abandoned when things get really tough then it is at best a more limited principle than it first appeared.

Possibly (I am unsure) considering a relative vs a stranger has introduced something into the thought experiment that changes the nature of the dilemma. I would certainly agree that if the two children were strangers I would almost certainly just toss a coin, regardless of their characteristics. It still seems inescapable that I do NOT regard the rest of the world as equal in all respects to my close relatives. And that makes me wonder about how I feel about my friends, other Londoners, other people who ride motorbikes, etc etc.

Equality as between people I do not know and don't much care about except in an abstract sense doesn't seem like much of a moral principle to me.