This is a horrendous predicament, butr consider: If their kidneys are so difficult to match, isn't it likely that at some point a single kidney might become available from some other donor? How do they decide who gets it in that case? Isn't it the same decision? Would she turn it down because it would only save one of the boys?
She said she would then give a kidney to the other boy.
I'm with the other posters in saying that I think I would have to give up both of mine and go on dialysis myself.