Omaenaideyo (1987)

Nishimaru Nao (Saito Yuki) lives temporarily at the home of Hoshino (Hayashi Ryuzo), a university professor who is an old acquaintance of her mother (Aki Yoko) after the dormitory she was living in burns down and is being rebuilt. However, Hoshino's wife has already passed away, and the family has four sons, the eldest Hoshino Kyohei (Nukukawa Toshikazu), the second son Shinpei (Sawamuki Youji), the third son Yohei (Osawa Hidetaka), and the fourth son Junpei (Nakajima Daisuke), so that they do not make any mistakes and do not see Nao as a member of the opposite sex, Nao enters into a fake marriage and lives in the house as the wife of the family. However, the four brothers are curious about their new mother, Nao, and soon a love affair begins...