In the novel A Pocket Full of Rye, Mrs Sparrow is mentioned by Miss Marple. She had a son who was a naval officer. When he had pneumonia a nurse helped him to recover. They fell in love and were married and had two children.
The nurse had previously taken care of Miss Marple, when she had broken her wrist.
Miss Marple mentions Mrs Sparrow and her son when she is talking to Jennifer Fortescue, as Jennifer had nursed Percival Fortescue when he had pneumonia, and they had also gone on to get married.