While their parents were away, the protagonist, Itsuki, was asked to look after his younger sister and returned to his family home.
There, he found his sister, Hinano, lounging lazily on the couch as usual, playing video games.

Itsuki scolded his messy sister but still took care of her.
Hinano pouted and whined a little, but she still clung to her older brother, acting spoiled.

Their sibling relationship seemed unchanged.
Nothing out of the ordinary, and the two continued living together peacefully—

"Wait… huh? Were we… always in this kind of relationship…?"

…Or so it should’ve been.

But somehow, a mix of impulse and stubbornness led the two to cross a line they shouldn’t have.

The next day, Itsuki, overwhelmed with guilt, became even more indulgent toward Hinano.
But Hinano, on the other hand, didn’t seem bothered by what had happened at all.
In fact, she smirked as she watched her brother’s flustered behavior.

(…Hmm… if I do that kind of thing with him, will big brother be even nicer to me?)

Her thoughts took a wild leap, and she began clinging to him more intensely—and more suggestively—than ever before.
Itsuki, determined not to let things spiral further, tried to resist.

But will he be able to fend off her temptations and set his lazy sister back on the right path…?

[Translated from dlsite]