Koakuma na Osananajimi ni, Itadakaremashita. Bed no Uede

Chiyoko, an ordinary office lady, always took care of Mutsumi, a younger boy she met in elementary school. She always felt a little inadequate around Mutsumi, who lived a glamorous life, because she was just a "caring older sister" to him. When Chiyoko went to Mutsumi's house to help her dress up for a company party, she got an unexpected reaction..? (Source: S*Girl, translated)