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When your girlfriend first insisted you call her your little sister, you weren't really sure how to process your feelings.
Burying your head in your hands, thoughts a mess, a decade of suppressed fantasies unraveling all at once.You... wanted this? Why... why did you want this? Why did the idea of calling her 'sis' make your heart race? Had she really figured you out so quickly? or was this her own idea...? What would people think, if they found out? You muttered out a noncommittal answer. You needed to sleep on this, think it through.
The next evening, you weren't really sure how to start talking. You joined the call and sat on the couch, staring out the window, too embarrassed to bring it up. She broke the silence for you.
"Did that... really happen, last night? I didn't just dream about asking you that, right?"
Fuck. It was real. "Y-yeah, it happened... s-sis." Flustered, you both devolved into the verbal equivalent of a keysmash.
"Why does it feel so... good?"
A question without an answer. You let out a "yeah..." that was more of a relieved sigh than a word.
You paused for a moment, pondering that question. There was something special about the intimacy of someone you've known almost your entire life... and things had been going so well with her that if you suspended your disbelief, just a little, it started to feel like you had known each other for years. Calling her 'girlfriend' had never felt quite right, anyway. It was as if you had wanted to be something more all along...
Unfortunately, you were a little distracted by the sudden recontextualization of all of the lewd things you wanted to try together with your new sister. Breathing into your sister's ear. Biting your sister's neck. Pinning your sister's arms down to the bed as she struggled, hoping you'd apply just enough force to let her feel helpless, but able to squirm in response for you. Feeling your sister's lips hungrily trailing along your shaft.
You accidentally let out a moan, and the voice on the other end of the phone immediately got excited. You blushed, feeling your ears turn red.
"Aww, my little sister is so cute." You watched as she reached her limit under your touch, moaning and panting and quivering for your eyes only.
"Sis, I... I love you."
You smiled, wiping the last of your little sister's cum off of your hand and lying down. "I love you", you replied, weaving your legs together and wrapping an arm around her from behind. "Now get some sleep, okay? I'll be right here next to you in the morning."
Nothing made you happier than looking after her. After all, you were her only big sister.