My Classmate Is Dad's Wife (2007)

1st Episode: Junichi's father, Yoshiyuki is a professor of the university that Junichi goes to. Yoshiyuki has remained single since Junichi's mother died 10 years ago. One day, Junichi learns that his father is getting remarried; surprisingly his stepmother turns out to be Aya, the classmate he secretly admires. How will their relationship become? 2nd Episode: Naoki and Maki are non-blood-related siblings who study in the same university. When undisciplined Naoki is retained due to poor academic result, Maki becomes a senior of her brother. Since then, Maki begins to act harshly to her brother in order to help him get over his psychological barrier to learning. However, being together day and night, Naoki finds himself in love with Maki gradually..

Unfortunately the movie My Classmate Is Dad's Wife is not yet available on HBO Max.