Mesmerized is a 1986 film starring Jodie Foster and John Lithgow.
Set in early 20th century New Zealand, the story begins with a baby being dropped off at a Foundling Hospital by her grandfather.
18 years later, Victoria (Jodie Foster) agrees to marry businessman Oliver Thompson (John Lithgow).
Because she is not yet 'of age', Victoria is sent to school.
Unhappy soon after she is reunited with Oliver, Victoria runs away with Oliver's youngest brother, George (Dan Shor). After Victoria is returned home, she is told that George has died.
As Oliver suffers from poisoning (initially caused when he's killing rats), Victoria soothes him with mesmerism and cholorform. She is charged with his murder but is acquitted.
Mesmerised is apparently loosely based on the Pimlico Mystery wherein Adelaide Bartlett is acquitted of the murder of her husband in London https://en.wikipedia.org/wiki/Pimlico_Mystery