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Have you tried to talk to her midwife if she wants to give a child for your wife's stead? It was an Oriental custom that is mentioned often in the Bible and many godly men did that. Not that it is pleasing in the eyes of our Lord, but these are extraordinary circumstances. Now, I know we aren't in an age of midwives and servants, but it could be an option, even though the consequences could be quite destabilizing in your future household.
In any case, I am very sorry for your fate. If it is God's will to not bless you with children, then He will bless you in another way - I am sure of it.