Mary Cavendish is the wife of John Cavendish, the stepson of Emily Inglethorp.
In The Mysterious Affair at Styles 
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Mary added a sedative to Emily’s coffee. When Emily died, she thought she was the murderer but later realized Emily was poisoned with strychnine.
In Curtain Edit
In Curtain, it is revealed that John died before Mary and that the latter went to live in Devonshire.
Portrayals Edit
In Agatha Christie's Poirot, Mary Cavendish was portrayed by Beatie Edney.
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