Blooming Early (1976)

Original title: 感じるんです

There is a girl who is a student at a tailoring school. She has a friend who has a boyfriend and is always having sex. She herself, however, remains a virgin. She sees her brother masturbating one day. She asks her friend to have sex with her brother. Her friend asks her to spread it for her older brother in exchange. It is a deal and the four go on a hike.