My stepmother's burden is my ex-girlfriend.

Yes, this is yet another trap set by fate.

This woman and I happen to share the same birthday.

Although that's not why we became close, I do remember saying disgusting things like, "Then we can celebrate our birthdays together!" and even going through the wicked ritual of exchanging gifts. Although I've locked that memory away and thrown it in the trash.

"So there shouldn't be any distinction between elder and younger siblings between us, right?"

"How come I remember you just loudly proclaiming that I'm your younger sister?"

I only said that because, compared to my stepsister, my stepsister seemed more agreeable; I didn't mean anything else.

"In any case, this premise will not be shaken. Because we only share the same birthday—but our birth times are different."

"Time of birth?"

"I've already found out."

The woman spoke in the tone of a detective, showing me the screen of her smartphone.

"Look."

The smartphone screen displays a photo of a baby. It appears to be a photo of an album page, with text written below the photo.

"Your birth time is 11:34 AM."

She swiped the screen to switch to the next picture. It was also a picture of a baby, and the woman pointed out the clock in the photo.

"Then, according to this photo, I was born at least before 11:04 AM. I was born at least half an hour before you, get it?"

Is this guy serious?

For such a trivial matter, you even dug out my family's photo album to investigate?

"I think you're crazy."

After I shared my honest thoughts, Jie suddenly blushed.

"Wha...why? Perfect reasoning requires complete evidence!"

"Here we go again, you die-hard fanatic of classic mystery novels. If you value jigsaw puzzle-style reasoning so much, why don't you just stick to playing jigsaw puzzles?"

"Whoa! You're picking a fight! You're picking a fight with the entire world of classic mystery novels! Come on, let's argue!"

"Alright, if I'm going to play a game with someone like you who loves to argue about fairness but never does any reasoning before the resolution, I'm sorry, your argument has flaws."

"What loophole?! I think your eyes are the real holes!"

Faced with a fake mystery fan who went into a rage mode after being hit where it hurt (the kind who deliberately ignores the challenge I gave to the reader), I presented counter-evidence.

"You take the premise that 'the first one born in this world is the older sister or older brother,' but there's a fallacy here. In Japan, since ancient times, twins have always been considered the younger brother or younger sister because the first one was born first."

"Huh? Why?"

Jie Nu showed a simple and interested expression, tilting her head slightly.

"There are many theories. Some say the firstborn makes way for their older siblings, while others say it's because the later-born is positioned higher in the womb. But in any case, if we consider us, these two step-siblings born on the same day, as step-twins, then you, being the firstborn, are the younger sister. Okay, do you have anything to say in your opinion?"

"But...but we're not twins..."

"If we're going to get to the point, then we're not even brothers and sisters, just two burdens."

"Waaaaah..."

The girl growled resentfully, glaring at me with eyes full of hatred. Haha, obediently submit to me now.

"...Wait a minute?"

"No way, get out of here!"

"What you just said about the order of twins, is that something from the past? Nowadays, isn't the firstborn always the older brother or sister...?"

"...Tsk! Just be obedient and you'll be fine."

"Ah! So, you were planning to set me up!"

"Anyway, I'm the older brother. Okay, QED (proof complete). Dismissed."

"I'm the older sister! Asking me to be your younger sister would give me the creeps!"

We stared at each other, face to face. To describe it as sparks flying between our gazes would be a gentle way of putting it. To me, it felt like our gazes were slashing each other like something out of a Futaro Yamada anime, splattering blood.

From the increasingly menacing gaze of the woman, I saw that Amakusa Shiro or someone else was about to be resurrected through the demon world, so I sighed and stopped arguing with her.

"...Continuing to stare at each other isn't a solution. At times like this, finding a game to compete against each other is the behavior of a rational human being."

"Although his tone is unpleasant, he's right."

"What kind of game should we play? Rock-paper-scissors, drawing lots, or flipping a coin?"

"Wait a moment."

"No way, get out of here!"

"Don't automatically respond to this kind of talk!"

Oops, I forgot to turn off the bot.

The girl covered her mouth with her hand and said, feigning intelligence, "Let me think..."

"...How about we take this opportunity and do it this way?"

"I really want to completely reject your opinion, but fortunately I am a very rational person, so let's hear what you have to say."

"This is infuriating... From now on, we have to hide our true relationship and pretend to be step-siblings with a fairly good relationship, right?"

"It is extremely regrettable."

"Although nothing has happened yet, one of them might slip up sooner or later—in other words, they might do things that step-siblings shouldn't do, right? So whoever does those things loses, how about that?"

"Hmm...can you do it?"

"What can or can't be done?"

"According to this rule, I'm definitely going to win."

"I think you're treating me like an idiot!"

It's just a logical deduction based on the facts.

"...Okay, let's do it this way. This will create tension, which will help us hide our relationship... By the way, does this rule also apply when Dad or Aunt Yu-ren isn't around?"

"Of course. It applies right now as well."

"I see. 'Whoever does something that step-siblings shouldn't do will be treated as a younger brother or sister,' right?"

"Each time you lose, you only get to be the 'younger brother' or 'younger sister' once. How you play the role specifically depends on the situation each time."

"There's no point in making a single mistake that you'll regret forever, right? Okay, that's decided."

"Alright, let's begin now!"

*Clap!* The girl clapped her hands—and then something happened.

Jie quickly moved to my bookshelf and began picking out books with an air of nonchalance.

"Wait... why are you going through my things without permission?!"

"Huh? What's so strange about that? We're brothers and sisters!"

Seeing the woman's sly smile, I finally understood the true meaning of this regulation.

Even if it's clearly a prank, if it's something that's generally considered acceptable between siblings, you can't just say no without a reason. Because doing so would be considered "inappropriate behavior between step-siblings."

In other words... this regulation is essentially a get-out-of-jail-free card for pranks!

This woman…! So that's why she proposed this rule! She's wicked to the core! If any man falls for such a wicked woman, he must be a naturally eccentric person himself!

...This is terrible.

As I glared at the woman who casually picked up books from the shelf and muttered "Oh—", "Wow—", or "Ugh," a growing sense of crisis arose within me.

Having my bookshelf ransacked feels like having my inner thoughts spied on—very uncomfortable, but thankfully I don't have anything I'm afraid of being seen. At most, it's just some light novels with a touch of eroticism.

The problem is... the drawer of the desk next to the bookshelf where you do your homework.

That drawer was practically the only Pandora's box in my room. Inside were my notes from a novel I wrote during junior high, something I bought at the pharmacy for some reason—and a gift from this woman herself when I was dating her!

The thought that she might find that thing—

"Oh my god, you still keep this kind of thing around? Could it be that you still have feelings for me? What—? Please don't do this—! That's disgusting—!"

—She absolutely must not be found.

If this continues, Jie's interest will eventually shift to the desk. I need to find a way to attract her attention before that happens. And it has to be something that feels completely natural for step-siblings!

I mobilized every brain cell to find a breakthrough. This was probably the most mentally taxing thing I'd done since my high school entrance exam.

The effort paid off—I've come up with another application of this "sibling rule".

"Please don't do that."

The fragile tone in my voice made Yui turn her head, her black hair swaying.

I got up from the bedside and walked over to her. The woman looked confused and looked up at me.

"I never want to be like this with you again, hating each other..."

“Eh…”

The girl slightly widened her eyes. My serious expression was reflected in her eyes.

"If you're not satisfied with me, I'm willing to apologize and won't appear before you. So... let's stop here."

I put my hand on Jie Nu's shoulder and told her in my most serious tone.

The girl's gaze wandered around, then she looked up again and glanced into my eyes.

Her large eyes shimmered slightly. She gazed at my face with a dreamy look, the confusion gradually fading.

Finally, her eyes, fixed on my stern expression, slowly focused—

"...Irito-san..."

"Okay, out."

"what?"

Facing the stunned and frozen-on-the-spot-mouthed woman, I smiled at her with ill intent.

"Brothers and sisters do not address each other by their surnames."

The speechless girl's face gradually turned red, like hot water steeping a tea bag.

Intentionally reminding the other person of past relationships—it seems she has finally discovered that this is the sure-fire way to win this rule.

"You...you...you can't do that...then you're out too!"

"Where am I out? It's perfectly normal for me not to want to be enemies with you, isn't it? Because we're brothers and sisters."

"Aaaaaaahhhhh...! Waaaaaaaahhhhhh...!"

I looked down at my stepsister with satisfaction, who was blushing and holding her head in frustration.

"Alright... as agreed, you can be my stepsister, okay?"

"Wha...what do you want to do...!"

"Why are you hugging yourself and backing away? What do you think of your stepsister?"

I do intend to thoroughly humiliate her, but I won't go too far. Next time, I'll make her my cat-eared stepsister's maid.

"Since this is the first time, let's keep it simple. Change how you address me."

"How...how should it be changed...?"

"Whatever."

Let me see what your stepsister is like, you bastard! Hahaha! That's so satisfying! (Opens mouth wide and gulps down red wine)

"Ugh..." Jie Nu groaned in extreme dissatisfaction, her eyes wandering aimlessly, clutching her soft, clenched hands to her chest—her face flushed with shame as she looked up at me.

A faint, trembling voice rang in my ears:

"Brother... Brother, brother..."

"........."

I turned my face away.