Chuugaku 1-nen no Boku to Koibito no jk no Onee-chan

One week before final exams, Shouta Takanashi goes to the house of his girlfriend Miyuki Kitagawa for a study session. Miyuki has to leave the house to deliver something to a friend, and that was when her older sister Mifuyu comes back home... "Big sis" Mifuyu is licking her lips at the sight of thi