«Who is it...?» Rika asks in a lazy voice after hearing someone knock on her bedroom door.
She's wearing her school uniform's thigh-high tights, a large white t-shirt that almost completely covers her butt – her white panties peek out from underneath – and she's completely absorbed in reading a thick book with a purple cover and black metallic edges.
«Who do you think it is? There are only three of us living in this house right now, and Hakuto went out to buy stuff for tonight's dinner!»
Yuzu's voice comes from the other side of the door, which she swings open even before getting permission – and yes, Rika takes her privacy very seriously, so much so that she made both her sister and her mother swear to always knock first.
«What do you want?» Rika asks, clearly annoyed by the interruption, without even lifting her eyes from the book.
«Do I really need a reason to want to spend some time with my beloved big sis?»
«Yes.»
«Ugh... FINE, I'M COMING!»
She doesn't even finish the sentence before throwing herself onto her sister. Yuzu is wearing a similar outfit to Rika's, but without tights, and both her t-shirt and panties are black. She dives onto Rika's back and ends up lying fully on top of her, her breasts squished against Rika's shoulders.
«Can't you see I'm studying...?» Rika grumbles, flipping through a yellowed page filled with strange geometric shapes and symbols that don't seem to belong to any modern language.
«Wow! You've gotten this far already?! You're amazing, big sis!» Yuzu exclaims in awe, noticing that Rika has already read more than half of that strange book.
«No, you're the slacker...!» Rika replies, closing the book and placing it on the nightstand next to her bed. «Just tell me what you want, so I can go back to studying in peace... and move your boobs off me, they weigh a ton!»
Without saying a word, Yuzu lifts herself up and sits directly on Rika's large, soft butt. Then she starts bouncing on it like she's on a hopper ball, so energetically that the mattress begins to bounce along with her – just like Yuzu's boobs.
«Y-Yuzu... stop it!» Rika exclaims, her voice shaky and unsure.
«Being a woman is great and all, but there are moments in life when having a nice big dick like Hakuto's would really come in handy... and this is definitely one of them!»
«Y-You... h-how the heck do you even know that...?!»
«Oh, who knows... maybe because I slept with him last night?»
«Y-You... d-did... what...? Y-You seriously have no shame...!» Rika pants, her face turning bright red.
«What's so wrong about sleeping with your stepbrother's dick tucked between your boobs...? I read online that it's a super common way these days to deepen the bond between two people. But you wouldn't get it... you're so old-fashioned!»
«I-I seriously doubt that's true... a-anyway, stop saying stuff like that! It's really embarrassing...!»
Yuzu suddenly stops bouncing. She presses her crotch firmly against Rika's butt, sinking into her soft cheeks, and starts rocking her hips back and forth in an increasingly fast rubbing motion — and as the intensity grows, so do Rika's moans and gasps.
«Why do you keep rejecting your true nature, big sis? Until you accept it, things like what happened today at the swimsuit shop or in the school hallway won't stop — they'll just keep happening more and more as time goes on!» Yuzu's voice turns strangely serious, despite her flushed face, aroused by the intense rubbing of her pussy against her sister's butt. «I-I... I can't keep living in fear that something awful might happen to you at any moment, just because you stubbornly refuse to accept who you are! Today it was that fat guy — but what if next time it's someone even more dangerous? I won't just stand by and do nothing while my big sister keeps putting herself in danger over and over!»
Yuzu's words are strong and determined now, but at the same time desperate and trembling with sobs. Big teardrops fall from her eyes, landing on Rika's back.
«I-It doesn't matter how... even if I have to use force, I'll make you do it... We already lost our father... I don't want to lose you too!»
«Yuzu...»
Rika rolls onto her back — now lying face up, with Yuzu still sitting on top of her. She reaches out to her little sister, grabs her by the shoulders, and pulls her close into a tight hug — a hug full of love and warmth, even as Yuzu's tears keep falling without end.
«Have faith in me, little sis,» Rika whispers gently, brushing her thumbs under Yuzu's eyes to wipe away the tears. «I'm sure I'll find a way to solve this problem before long... even without doing that thing. I promise.»
Then, Rika's lips touch Yuzu's — a quick, fleeting kiss on the lips.
«A... kiss? You do know what a kiss to seal a promise means, right...?» Yuzu murmurs, trying to smile.
«Who do you think I am? Of course I know! As if I wouldn't know the traditions of our ancestors!» Rika replies, slipping back into her usual tsundere tone.
«Now, if you don't mind, I'd like to get back to studying! Go back to the living room — Hakuto should be back any minute — and closethedoor... FORREAL, THISTIME!»
Yuzu jumps off the bed, snapping to attention like a little soldier — she even throws in a military salute.
«Yes ma'am!» she exclaims with a burst of renewed enthusiasm.