The Devil's Sisters(1966)

This film is based on the Las Poquianchis case in Mexico where Delfina and María de Jesús González lured in young women with false help wanted ads. They would get the women addicted to drugs and put them to work as prostitutes. Once they became too ill, used up or stubborn, they would be killed. Customers may also be killed if it appeared to be profitable enough. When police officers searched the sisters' property they found 91 bodies.