My Stepmom's Daughter Is My Ex - Season 1 - Eps 1: The Former Couple Refuses to Say...
Mizuto and Yume used to date, but now they've suddenly become stepsiblings. Both of them agree that they don't want to do anything that'd disappoint their newly remarried parents. However, neither of them can accept the other as being the eldest sibling in the relationship. So they both agree to settle the matter with a game. Whoever does something in conflict with what people might consider normal sibling behavior, loses. Making them the younger sibling.
About My Stepmom's Daughter Is My Ex

Title: My Stepmom's Daughter Is My Ex
First Air Date: 2022-07-06
Last Air Date: 2022-09-21
Status: Ended
Rating: 6.4/10 (from 51 votes)
Language: JA
Seasons: 1
Total Episodes: 12
Network: AT-X
Genres: Animation, Comedy
Production Companies: project No.9, ABC Animation, Bit grooove promotion, Pony Canyon, APDREAM, Happinet Phantom Studios, DREAM SHIFT
Synopsis
Ah, high school. Is there any better place to start fresh after a horrible middle school relationship? Nope! Not unless your ex ends up at the same school as you and is now your stepsibling. What was supposed to be a sanctuary of peace where I could avoid ever seeing her again has become a living nightmare! Everywhere I look, I see her—in my house, in my school, in my class. There's no escape! She even claims that she's the older sibling. Like hell she is! But I won't lose to her. After all, I'm the older brother in this new family situation. That's right, we're family now. No matter how much we may have thought we loved each other before, we saw one another's true colors and realized we weren't meant for each other. That's why even though we may keep up a buddy-buddy sibling act for the sake of our parents, things will never go back to the way they used to be.
Cast

Hiro Shimono
Mizuto Irido (voice)

Rina Hidaka
Yume Irido (voice)

Ikumi Hasegawa
Akatsuki Minami (voice)

Nobuhiko Okamoto
Kogure Kawanami (voice)

Miyu Tomita
Isana Higashira (voice)