I think the real issue is which person has the most to offer society (though that includes them too). If the child is still conscious enough to live an enjoyable life, and is capable of bringing happiness to others through more than just his physical existence, I would say him. Otherwise, the child is just a sap on his parent and society. We cannot know what the young man will do with his life, only that his existence probably improves the life of his wife and child. He may change he may not- his past actions may not be an indication of his future ones. The doctor probably has the most to offer in the short term- but how short? If her continued existence helps others who would otherwise not be helped, and she is not in pain/ incapacitated, then she may 'deserve' life the most. From the information given I see no reason why the nun should live- surely she does not fear of meeting God, and without wanting to make further assumptions would probably find it most moral to allow another to live in her place.
Too much guesswork!