Suikyo no Shizuku - Seiin Shimai | 水鏡の雫 性淫姉妹

One day, the lead character (you) gets a letter from your former teacher disappeared over decades ago..... In the letter the teacher says that he needs your help. You rush to his place with the aid of the address on the letter, and finds a mysterious mansion on the edge of cliff. .....Yes, this is the start of the game....