The younger sister made a very wrong decision when she brought her boyfriend home while the house was full of very promiscuous older sisters, especially the housemaid. It's hard to blame the boyfriend in this situation as from the beginning to the end, he was constantly being provoked by the housemaid and the older sisters to seduce him. The story might have ended if one of the older sisters hadn't caught him and the housemaid being intimate. After being threatened by one of the older sisters to tell the younger sister about the affair, he was very scared. In a moment of desperation, unable to control himself, he forced himself on her, then proceeded to do the same to each of the older sisters. But it seems like all of this was part of the promiscuous women's plan from the beginning, as when he forced himself on them, they did not resist, but rather seemed very satisfied and even more enthusiastic during the act.