Ordeal by Innocence by Agatha Christie

When a man is convicted of murdering his adoptive mother, new evidence later proves he was innocent—but it is too late, as he has already died in prison. Years later, a mysterious stranger arrives, reopening old wounds and forcing the family to confront uncomfortable truths. Hercule Poirot becomes involved in uncovering a case where justice itself may have failed, and the real killer may still be living among them.