Yume’s face turned red upon hearing my unfiltered opinion. “H-How?! A perfect deduction requires there to be complete evidence!”
“There they are—the words of a true mystery fangirl. If you like puzzles so much, why beat around the bush? Go solve a real puzzle.”
“Now you’ve done it! You’ve just picked a fight with every true mystery fan out there! Are you prepared for that?!”
“All right. I’ll play along with your mystery game, even though you never come up with your own theories before the solution is revealed. As much as you fuss about what’s fair and what’s not, there’s a hole in your argument.”
“What hole?! You must be talking about those rotten holes in your head that you call your eyes!”
Just like all mystery fans (people who ignore any challenges to the reader), she entered her angry mode because I was right. And so, I presented my rebuttal to her theory: “In your argument, you stated that in this world, the firstborn child is the older sibling. However, that’s just a fallacy. In ancient Japan, when twins were born, the one who came out first was considered the younger sibling.”
“Huh? Why?” Yume said, tilting her head out of genuine curiosity.
“There are lots of different reasons, like the first child born acted as an usher to the other, or that the first child must have been lower in the womb than the other. Either way, in the event that we—stepsiblings who share the same birthday—are determined as steptwins, it would mean that you—the one who was born first—are the younger sibling. Got it? Good. Your counter?”
“W-We’re not really twins...”
“Yeah, and we’re not really siblings either. We’re just the children of our respective parents.”
Yume groaned in frustration in response to my logic, glaring at me spitefully. I wanted to laugh in her face and tell her to just bow down before me, but then she realized something.
“Wait a second.”
“Nope. Get out of my room.”
“What does our birth order have to do with ancient Japan? In modern times, the first one born is the older one.”
“Tsk. You should’ve just kept your mouth shut and let yourself be deceived.”
“Huh?! Y-You tried to trick me?!”
“At any rate, I’m the older sibling. QED. Case closed,” I said, standing up and glaring at her. “Now get out of my room.”
“I’m the older sibling! Just thinking about being your little sister makes me want to throw up!” She returned my glare.
It felt like sparks were flying between our eyes, although that seemed like a bit of an understatement. It was more like we were in one of Futaro Yamada’s ninja novels, furiously clashing swords with one another, blood spraying everywhere.
I let out a sigh and calmed myself as I began to feel Shiro Amakusa or some other warrior reincarnating from hell in Yume’s fierce gaze.
“Us staring at each other isn’t gonna solve anything. I think the rational decision is to settle this with a game or something,” I suggested.
“I don’t like the way you phrased that, but I agree.”
“So, what’s it going to be? Rock paper scissors? Drawing lots? Coin flip?” I asked.
“Wait a second.”
“No. Get out of my room,” I snapped back instinctively.
“Stop saying that automatically.”
Oops. Forgot to turn off my mental response bot.
“Hm...” Yume put her hand to her chin, trying to look smart. “I’ve got an idea.”
“I would like nothing more than to reject you outright, but lucky for you, I’m a rational person. I’ll hear you out.”
“You’re so annoying.” She rolled her eyes. “Anyway, now that we’re living together, we have to hide our true relationship and pretend to be siblings that actually get along, right?”
“Regrettably, yes.”
“We’ve been doing okay, but one day, one of us might slip up and say or do something that isn’t something that stepsiblings would say to each other, right? How about whoever slips up is the loser?”
“Hm, I see. And you’re okay with that?” I asked.
“What do you mean?”
“These rules make it clear that I’m gonna win.”
“You’re looking down on me!”
It might have sounded like that to her, but I was merely stating my logical theory based on the facts. “I’m fine with those rules. It’ll keep us on our toes and help us keep things under wraps. By the way, do the rules count when our parents aren’t around?”
“Of course. The rules even count right here, right now.”
“Uh-huh. So the one who doesn’t act like a stepsibling is the younger one, huh?”
“It’s an ongoing game where if you slip up, you lose, and the winner can decide how they want to be called until the next slipup,” Yume explained.
“Makes sense. It’s no fun if it’s just a onetime game of sudden death.” I nodded. “All right. I’m in.”
“Good, then our game starts now!” Yume said, clapping her hands together.
Not a moment later, Yume moved to my bookcase and nonchalantly began rummaging through it.
“H-Hey, what are you doing?!”
“Hm? Just sibling stuff. Nothing weird,” she said with a devilish smile. As I looked at her, it began to dawn on me what the true idea behind these rules was.
Even if one of us did something to get a rise out of the other, we couldn’t adversely react to anything that was deemed normal for siblings, because otherwise that person would be the one not acting like a stepsibling. These rules were just an excuse for harassment!
Th-This girl! She suggested these rules for this very reason! She’s rotten to her core! Whoever falls for a girl this ill-natured has to be just as twisted!
As I stared at the girl in front of me who was making various reactions at the books she randomly pulled off my shelf, I knew that I was in a bad position. I could sense that I was in danger. As she went through my books, it felt like she was peering into my soul. It made me slightly uncomfortable, but fortunately, the slightly erotic light novels that would make me really uncomfortable weren’t there.
The problem was my desk drawer next to the bookcase. It was the Pandora’s box of my room. Hidden inside were a notebook with my original novel written in it, a certain something I bought at the drugstore with mixed feelings, and a present that I’d received from this very girl when we were still dating.
I could already picture how she’d react. “Ew, you still have this? What? You still hung up on me or something? Huh?! You seriously need to give up, creep!”
There’s absolutely no way I can let her find it. Unfortunately, at this rate, Yume’s attention would naturally move to the desk. I had to do something to draw her attention away from it and do so in a way that was normal for siblings!
I kicked my brain cells into overdrive to look for a way out of this situation. This might’ve been the hardest I’ve thought about anything since the entrance exam for high school. My desperate thoughts brought me to a part of the “sibling rules.”
“Please, stop,” I said in a feeble voice. This caught Yume’s attention, and she spun around to face me, her black hair fluttering in the air.
I stood up from the bed and walked towards her. As I grew closer, she looked up at me, her face becoming increasingly filled with confusion.
“I don’t want to fight anymore,” I said.
“Huh?” Yume’s eyes widened and reflected my meek expression.
“I’m sorry if I made you angry. I’ll leave if you want. Just... Can we please just stop this?” I put my hands on her shoulders and spoke in a clear, serious tone.
Yume shifted her gaze but quickly looked back into my eyes one more time. Her large, doe-like eyes trembled slightly. As she gazed at my face, the confusion from her face slowly dissipated, and finally, she was firmly focused on my expression, and—
“Irido-kun...”
“Aaand you’re out,” I said.
“Huh?” Yume’s mouth was agape.
I smirked. “Siblings don’t call each other by their last name.”
Yume started out dumbfounded, but her face slowly grew redder and redder, like water after you put a teabag in it. Bringing up anything that might remind us of our past relationship was forbidden under the rules and therefore a surefire way to win, and it seemed that she’d finally realized this too.
“Wha— No, th-this can’t— You’re out too!” she stammered.
“How? Not wanting to fight is a very normal thing, isn’t it? We’re siblings, after all.”
“Agh! Waaah!” My little stepsister clutched her head out of vexation, blushing up to her ears while I watched from above, satisfied.
“All right, time for your end of the bargain. Ready to become my little sister?”
“Wh-What are you planning to do?!” she cried, shrinking away from me.
“Stay there and stop holding yourself. That’s not what little sisters do.”
Every last part of my being wanted to humiliate her, but I thought better of it and decided that I would save the cat-eared maid stepsister punishment for another time.
“Let’s keep it simple since this is the first time and everything. You’re going to change the way you address me.”
“H-How?”
“However you want.” Internally, I felt like a king. Ha ha! Show me what you believe a little sister is. Ah, delightful. Wonderful! My internal self downed a glass of red wine.
In response to my order, Yume averted her gaze and let out an extremely displeased groan while bringing her clenched fist to her chest. Before long, she looked up at me, her face as red as a baby’s from embarrassment, and in a small trembling voice, her words echoed in my ears.
“O-Onii-chan...”