The Lemesurier Inheritance

The Lemesuriers are supposed to be under a medieval curse derived from a medieval ancestor of the family - he suspected his wife of being unfaithful and that their son was not his own. As punishment he murdered them both and dying she cursed his family to never have a first-born son live to inherit. Hercule Poirot and Hasting meet the latest heir, an eldest son, but he apparently commits suicide that night. After the death of another heir, the mother of the next heir asks Poirot to protect her son, who has suffered some potentially fatal accidents. Can the fatal curse be true? Or can Hercule Poirot solve the riddle on time?