Is what the wife did considered a mistake when she concealed from her husband her illness and her need for surgery, despite informing him of her visit to the doctor, under the pretext of not wanting to bother him while he was traveling?

The wife should have informed her husband about the test results first. However, since the matter has already occurred, the wife is advised to forgive his reaction and think well of him. The husband is advised to be gentle with his wife and appreciate her intention. Furthermore, the sister is not a mahram, but her presence negates khalwa when non-mahrams are present.