Despite knowing that deceiving their mother and betraying her trust is immoral and would induce guilt, Liyu and the other resolutely embark on a daily life of surreptitiously eliminating the chosen ones for the sake of keeping their mother alive.
In order not to arouse their mother's suspicion, Sebastian intentionally trains all of them, excluding the eldest sister, to improve their acting skills.
This way, they can create a false information world around their mother, and their clandestine activities can pass off unnoticed in front of her.
However, after all the training, they identify a serious problem. There's a massive loophole in their false information world, one that can expose them all and get them severely punished at any moment. That loophole is their naive sister, Limo.