Adult PC Game: Ore to Nakayoshi Shimai no Inma na Himitsu by Ammolite Archive

Without asking for his opinion, the protagonist's father announces that he is going to remarry.
As such, "Kousuke" has to suddenly live with his two new step sisters.

Although the sisters have opposite personalities, they get along very well.
Kana, the older sister, calls herself Kousuke's "onee-chan" despite being younger than him, and has a very sunny and mature disposition.
Saya, the younger sister, hates men and does nothing but glare at Kousuke when he's around.

While Kousuke is still confused about the sudden developments and change in his life, his father moves to a new house with his new wife only a month later. As a result, Kousuke has to take care of his sisters. Luckily, as the sisters get along well with each other, all Kousuke needs to do is do the housework and manage the money their parents send them.

And so, their peaceful life together begins. A few weeks later, Kousuke wakes up in the same bed as Kana...