<bracing for impact of angry replies from people really, really, really, close to their parents>
Since your m-i-l explicitly has told your wife to stay on her holiday, I think you should. Is there anything your wife needs to do for her mother during the day of surgery that the rest of the family can't? Just keep in contact by phone and make up for it by visiting when you get home.
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(You can find good discussions on topics like this in the Women Travelers forum.)