The Lemesurier Inheritance

We are told that the Lemesuriers are under a curse that can be traced back to a medieval ancestor. He suspected his wife of being unfaithful, and that their son was not his own. As punishment he murdered them both. While dying, she cursed his family to never have a first-born son live to inherit.

In modern times, Hercule Poirot and Hasting meet the latest heir, an eldest son. But later that night he apparently commits suicide. After the death of another heir, the mother of the next heir asks Poirot to protect her young son who has been having accidents that could have been fatal. Can the ancient curse be true? Is there a riddle which Hercule Poirot can solve in time to save her son?