Three months have passed since the sister began living with her unemployed brother-in-law and younger brother. During this time, the younger brother had been paying her a small allowance as thanks for helping with housework. However, after witnessing her comforting his brother-in-law, the younger brother feels an unexpected arousal. One day, while cleaning his room, she pulls back the bed cover and accidentally discovers his erect penis, making her aware of his feelings. Later, during a drinking session, the sister finds out the younger brother has been spending huge amounts of money at brothels. Surprised, she says, "Huh? You're paying that much? What a waste! Just give that money to me instead, haha!" Caught off guard, he jokingly proposes the idea, to which she replies, "Hmm? If you're paying, why not? Let's test our compatibility!" Their playful banter quickly turns into mutual excitement, crossing boundaries as their relationship escalates beyond mere arrangement.