Mei Shoujo

The protagonist has no direction in life and spends his days indifferently attempting to make ends meet. His sole possession is the home his parents left him.

One day, he gets a phone call from his sister, asking him to look after her twin daughters "Aya" and "Saya." She wishes that they remain at the main character's residence, which is near to the "Twin Star Girls' Academy" (Sousei Jousei Gakuen) that they will attend starting in the spring.

The new everyday life between the main character and the two nieces begins. Nonetheless, the protagonist, who has a strong sexual urge, cannot resist the presence of the nieces. Forgetting his status as a relative, he plays pranks and seduces them while remaining courteous.

And the deeds get more heinous...

[Modified from Getchu]