The young man is familiar with the older sister but likes to fuck the younger sister

Today Yume's parents went on a trip, so her older sister took this opportunity to invite her boyfriend over to hang out. Yume, who was busy writing an essay, asked her sister's boyfriend - Tsukasa - for help. Tsukasa had previously stolen Yume's underwear and she found out about it.

So, if Tsukasa didn't want her to tell her sister about it, he had to agree to do that with her. And since then, the two have been secretly having a physical relationship, in the bedroom, bathroom, kitchen, even when the sister is nearby...