There is no logical reason for Mrs. White's bizarre request, especially after her husband warns her that Herbert
"... has been dead ten days, and besides he--I would not tell you else, but--I could only recognize him by his clothing. If he was too terrible for you to see then, how now?"
Mrs. White's insistence upon the second wish--that Herbert become alive again, regardless of his physical condition--comes from the love and grieving heart of a mother. An adult, Herbert still lived at his parents' home and was, no doubt, doted upon by his mother. She could not bear to be apart from him, and she was willing to accept her son back--in a "mutilated" state, if necessary.
"Do you think I fear the child I have nursed?"
It's an unexpected move from such a conventional woman. Her "foolish and wicked" decision could never have been conceived before Herbert's death, but now--with two more wishes to make and a broken heart to mend--Mrs. White takes the only step possible to reunite the family as before.