The brother should understand that the child is the most important one out of whom he can save. The wife wouldn't want to live knowing her child is dead because she lived and the child being a child would want to live simply because they are young and scared, plus they have a whole lot to live for.
I'd go on a limb and say most people would agree with me, but its just personal opinion and simple elimination. Once you have a wife and kid they are above the level of relationship that you and your brother would have.
Edit: who know's how x person would choose though and could completely change if actually put into this situation rather being a scenario