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 this naive boy who's just her type. This devilish older girl first checks to make sure that Miyuki is out of the house, and then makes her move on Shouta...!!

(Source: Melonbooks, translated)

Note: Released at Comiket 97.