In the Hercule Poirot novel Curtain, Margaret Litchfield was Matthew Litchfield's oldest daughter, sister of Elizabeth. Margaret was mentally manipulated by Stephen Norton into killing her tyrant father. Margaret later broke down and confessed the crime to the police. She was eventually claimed insane, and committed to Broadmoor. Miss Litchfield died there some months later.
Margaret Litchfield never appears in the novel.