amaenbo futago mama

Yuki has a beautiful mother, Shizuru. Since she works full-time, Miina, her younger sister, takes care of him and Naoto, his younger brother. One day, Yuki receives a letter from a girl. When he talks about it during dinner, Miina says, "Are you in love with a girl? Who is she? Please tell me later!" Yuki says, "Well, OK...." He is always confused when Miina asks him about it.... Why...?