Kazuya, the protagonist, is the middle child of three siblings.
His parents, both scholars, are often away on archaeology excavations, leaving him to handle the household chores in place of his lazy older sister and clumsy younger sister.
His days are filled with taking care of the house, dealing with his teasing older sister, Narumi, and looking after his scatterbrained younger sister, Chie.

Then, one night.
Late at night, Kazuya wakes up to a strange sensation in his lower body. When he lifts the blanket, he finds Chie sucking on his dick.
Stunned, Kazuya can’t even speak. Chie notices he’s awake but keeps going, her inexperienced technique still enough to push Kazuya—a virgin—over the edge.
He ejaculates almost immediately, and Chie chokes but manages to swallow it all.
Still in shock, Kazuya demands, “Why are you doing this!?”
Chie confesses that she’s always loved him. She even talked to Narumi about it, who advised her, “Just go for it and make him yours.”
When Kazuya protests that they’re siblings, Chie tearfully insists, “I know that, but I can’t hold back anymore.”
Moved by her tears, Kazuya gives in, and the two fuck.

The next day.
Chie’s behavior is noticeably different—brighter, closer to Kazuya than ever.
Narumi picks up on it immediately. Did something actually happen because of her advice?
The truth is, Narumi’s teasing has always been her way of hiding her own feelings for Kazuya. Now, those feelings start bubbling to the surface.

And so begins the strange love triangle between Kazuya and his two sisters.

[Translated from getchu]