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There are different possibilities of how Shirayuki will handle Haruki's sister.
Scenario 1: He silences the sister by plotting to kill her off.
Scenrion 2: He'll manipulate the situation if the sister ever snitches him to Haruki, and in which Shirayuki will use the sister to confuse Haruki by saying that her sister might have problems with him and that she's making up stories about Shirayuki.(which we know is a lie since he is actually a psycho lol)
Scenario 3: Shirayuki will find dirt on the sister and make her go to the other side(by psychotic and kill people as well). Like, Shirayuki will try to find people that the sister might have a grudge on and urge the sister to take revenge and thus the two of them will be put in the same situation(psychos who kill) and she will no longer be able to snitch on him since that'll put both of them in danger.
These are just possibilities lol.