A Blueprint for Murder (1953) on Netflix

Plot summary

Whitney Cameron reaches a hospital to find Polly, the stepdaughter of his brother's widow Lynn, in violent convulsions from an illness no one can identify. She seems to recover, then suffers a relapse and dies, and the doctors, who had guessed tetany, cannot say what killed her. Maggie, the wife of Cam's lawyer Fred Sargent, notices that the girl's symptoms resemble those of strychnine poisoning victims she has read about. Cam has to face the fact that his brother, Lynn's husband, died in much the same way, and he shrinks from accusing a woman he has come to love. Fred explains that the family fortune passes to Lynn only if both of her husband's children die, which turns Cam's worry toward Doug, his young nephew.