A Blueprint For Murder (1953)

Whitney 'Cam' Cameron returns to Manhattan to visit his widowed sister-in-law Lynn and her stepchildren, his niece and nephew. His niece has suddenly fallen ill with a mysterious malady whose symptoms mirror those of his deceased brother. After being hospitalized, the girl seems to be rallying but quickly takes a turn for the worse and dies from the undiagnosed illness. Cam's friend and lawyer Fred Sargent and his wife Maggie believe that the symptoms may suggest strychnine poisoning, and as Lynn stands to inherit a fortune if nephew Doug dies also, Cam now views her with suspicion. Despite circumstantial evidence that she murdered her step-daughter, Lynn is released for insufficient cause. When she tells him that she intends to take Doug to Europe, Cam realizes that desperate measures are needed if he is to save the boy's life.

IMDb: https://www.imdb.com/title/tt0045566