A 1930s double homicide in Le Mans France. Christine and Lea Papin, young sisters, live in maids for the same household, who brutally murdered the wife of the household and their adult daughter after almost 7 years of working for them.

Source information if you would like to read or watch for yourself

https://youtu.be/eWhqo_5gpOU

Christine and Léa Papin - Wikipedia

Papin Sisters: The Shocking Housemaids' Crime That Shook France (historicmysteries.com)