The question is, did Drury Lane cancel the performance. If so they I would question the charge. If they didn't cancel the performance they would be within their rights charge a fee for a reschedule. Its unfortunate for your sister but that's how its done.
You may want to post this as a review. More people will see it there rather than in the question section.
Even airlines don't do that when there's a major snowstorm. Shame on you Drury Lane.
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