In what year did Mary Slessor stop the killing of twins?

Mary Slessor didn't "stop the killing of twins" in a single year. Her work to end the practice of twin infanticide in the Efik and Ibibio tribes of southeastern Nigeria was a gradual process that spanned several decades. She arrived in Nigeria in 1876, and her tireless efforts, including rescuing and adopting twins, advocating for their rights, and educating the people about the value of all human life, contributed significantly to a change in societal attitudes.

It's important to note that the practice wasn't completely eradicated in her lifetime, and there were still areas where it persisted. However, her impact on the culture and the perception of twins is undeniable and serves as a testament to her remarkable life and work.