Hiiro no Shimai

The protagonist of the game is a college student named Shunsuke. His mother died when he was five years old. Now his father is marrying again. He and his new wife have departed on a honeymoon; while he is waiting for them to come home, Shunsuke discovers that he doesn't live alone any more: three young sisters of his stepmother have moved in. Shunsuke now tries to get to know them better (in every possible way..), at the same time gradually discovering dark secrets about his father's intimate life.

[From Moby Games]