Amnesia Labyrinth - v. 1

When high school student Souji Kushiki, an academic achiever and talented athlete from a well-to-do family, returns home from summer vacation, 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 mysterious conduct at first.
However, when Souji arrives at school the next day, he learns that, during the summer, three students were murdered by an unknown culprit. Are the victims linked, and could Souji's sisters have anything to do with the murders?
With the help of his cheery and spunky new girlfriend Yukako Sasai, straight-laced Souji is about to venture into a mysterious world that might just be the death of him.