Explanation:
Direct Speech:
The old lady said to her son, “Is it not your duty to help me in my old age?”
Indirect Speech:
The old lady asked her son if it was not his duty to help her in her old age.
Why?
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said to → asked because it is a question.
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Is → was (backshift in reported speech).
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your → his and me → her.
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if is used for a Yes/No question.