Little Sister (2020)

With her mother's remarriage, Dong-jun lives in his stepfather's house while they are on honeymoon. However, on the first night of living in his stepfather's house, he witnessed his step-siblings, Seol-ah and Do-jin, having sex. Seol-ah and Do-jin are not brothers or sisters, but Dong-jun is shocked at the thought that they are brothers and sisters. That night he dreamed for the first time in years and the next day, Do-jin leaves early and only Dong-jun and Seol-ah were left at home. Dong-jun feels burdened by Seol-ah’s kindness. After a while, Dong-jun, who returned from exercising in the yard, finds Seol-ah taking a shower with the door open. Seol-ah asks Dong-jun to wipe her back. Since they have become families, she suggested for them to shower together, and they had sex.