Amnesia Labyrinth

When high school student Souji Kushiki, an academic achiever and talented athlete from a well-to-do family, returns home from boarding school, he finds that his sisters Harumi, Youko, and Saki have changed. Their strangely clingy behavior borders on the inappropriate and bizarre, yet Souji just brushes off their odd behavior at first. However, when Souji arrives at school the next day, he learns that, during the summer, one of the students in his class was pushed to his death in front of a subway train. And what's more, two other students were stabbed by an unknown culprit. Are the victims linked, and could Souji's sisters have anything to do with the murders?