I am Sayuri Sato, 40 years old. - I have been married to my husband for 12 years. - I have no children and live with my husband and wife. - At that time, my younger sister came to live with me for a month. - Unlike me, my free-spirited younger sister loses the rhythm of her life by walking around the room completely naked in front of her husband. - Lately, I've noticed that my sister and her husband are getting closer. - Just then, I heard my sister's lewd voice coming from the room...