Murder on the Orient Express
Count and Countess Andrenyi
The beautiful Countess is the daughter of Linda Arden and the sister of the late Mrs. Armstrong. Like her mother, she is an excellent actress, though it's the grease smudge on the Countess's Hungarian passport that ultimately gives her identity away to Poirot. Her husband defends her innocence until the end, and he's telling the truth, since he did the stabbing for her.