The stepsibling dating fantasy is one that gets an understandable amount of mileage. Just among romantic comedies (particularly those aimed at the light-novel-reading otaku set) it pitches to a kind of ready-made "cohabitation with a significant other set up for you" appeal, with a dash of the taboo for flavor. And as with any played-out premise worth its salt these days, that means it's readily ripe for subversions and switched-around spins on its formula. My Stepmom's Daughter Is My Ex, as these stories are wont to do, pitches its particular point to you right in the title, taking things to their natural conclusion from there: What if the prospectively-romantic step-siblings had, in fact, already courted, dated, and broken up, and now just friggin' hated each other?

Much as I can personally see the entertainment value entirely contained in spiteful characters sniping at each other, it is fair to recognize that this might not be the endearing content other audiences would flock to such a novel for. Thus My Stepmom's Daughter clarifies the characterization of the titular exes extremely early: Mizuto and Yume at least have enough care for their now-coupled parents that they don't want to threaten it by bringing up their own sordid past. The writing also establishes the device of the two having kept their relationship a secret in a way that doesn't feel like too much of a contrivance for the sake of its gimmick, though readers might be asking some questions about it before they get to that explanation. Mostly, in spite of all the jabs Mizuto and Yume spend the book throwing each other's way, it actually helps provide a degree of genuine familiarity that you might not get in 'standard' romantic comedies, and their interactions make clear that the pair still have a degree of lingering attachment and affection for each other in spite of themselves.

That idea is reinforced by the actual structure of the book, as the narrative jumps between flashbacks to Mizuto and Yume's actual relationship and the recollections of it in the present day. The story's aiming to make a strong contrast between the rose-colored impressions of middle-school first love between two awkward nerds, and the cynicism that ensued for both of them as a result of that relationship going south. It's the whole range of emotions presented as part of the exhausting experience of growing into being a teenager, and honestly comes off pretty refreshing compared to other anime-adjacent romance stories featuring characters taking forever just to work their way up to asking each other out. Mizuto and Yume's relationship didn't just affect their connection with each other; it affected each of them personally, and the book revels in illustrating that formative aspect.

And the way it communicates that is through its other major structural gimmick: We don't just narrate the events through assumed audience-surrogate Mizuto, but spend 50% of the book on Yume's viewpoint as well! It's a great choice for a story like this, as it's able to highlight the disparities in the pair's experience in their coupled days, as well as adding a bit more suspense to the 'modern' parts of the story. Author Kyōsuke Kamishiro finds multiple opportunities to withhold information from one character's viewpoint, only to switch around and jump back with an illuminating explanation for maximum drama. Similarly, there are cuts between character's actions and reactions (particularly during the pair's date in the second half of the story) that well highlight the disparity in how they intend to act to each other and how their lingering feelings are actually affecting them. Yume in particular makes for a treat to read in moments like this, as her increasingly-irrepressible thirst for her ex contrasts with the more refined image we know she's putting effort into. It means the whole book takes on an effective "Will they/Won't they" setup less predicated on the step-sibling taboo and more on the question of if the pair have grown enough as people by this point that giving it another go might actually be a good idea.

All that framing and textual gimmickry (along with Kamishiro's generally strong prose full of knowing self-contradictions from the personalities of these unreliable narrators) makes My Stepmom's Daughter an entertaining read, even if the actual plot still comes off as an early kind of episodic. Most of the chapters in the first half focus on simple contrasts of the couple's dating days with their more hostile navigation of a current cohabitation-based situation. They're fun, but only come off as incrementally meaningful (though one Yume-viewpoint story focused on stolen underwear winds up a delightfully guilty pleasure on account of her increasingly-unhinged narration). So then in an apparent effort to kickstart some sort of escalation for this story, the plot suddenly drops a truly out-of-nowhere swerve in the book's second half, utilizing a previously benignly-established character. It does make for some functionally engaging drama, as the audience (and Yume) grapple with what exactly is actually going on, before the book uses that perspective-flip trick to have Mizuto clarify things. But it feels like a bizarre leap overall, especially when the ultimate purpose of this revelation clarifies itself as simply a way to facilitate Mizuto and Yume's 'date' late in the story, and the characters resolve themselves afterwards with little direct interaction.

It's conflicting, as that resultant date between the stepsiblings is probably the most entertaining part of the whole book. It makes liberal, effective use of both the flashbacks and perspective-flips that characterize this story. But more importantly, it starts to really address the complications of the main characters' growth in their new connection: That their cohabitation and status as adopted family actually motivates them to be more earnest and communicative with each other, and might overall be the key to getting over the shared spitefulness of their past. It feels like an effective analysis of real human relationships with an eye towards believable future developments, which makes it all the more unbelievable that it took the introduction of a left-field twist to bring it out.

Still, as a first volume, My Stepmom's Daughter succeeds by showing off its potential. It's got the chemistry between the two leads down, and that base energy, plus the actual writing thereof, can carry this well enough that I have to wonder how it'll work if it heads into deeper, more solid storylines. It makes for a few episodes of an entertaining romantic sitcom, only stumbling when it tries to find a way to accelerate some components of its story. I'd recommend it as a compelling curiosity of the romantic comedy genre, and as a light-novel whose gimmicky premise doesn't keep it from paying attention to more compelling character and relationship work.