Baka na Imouto o Rikou ni Suru no wa Ore no xx Dake na Ken ni Tsuite

One day, the protagonist who lives alone was visited by his younger sister, Hana. She came to ask him to help her study, but then she suddenly took off her clothes.

Unbeknownst to her, the protagonist got aroused when he saw her body, as she's the type of girl he likes. She also said, "I've been naked at home lately. It feels good, so please try it with me." Then proceeded to undress him and was surprised by his erect genital.

He couldn't tell his sister that it was because he's lustful. The protagonist told her that there is milk in there to make her smarter. She immediately grabbed the protagonist's rod and said, "I wanna drink Onii-chan's milk."

And so begins the lives of the protagonist and his idiot but cute little sister.

[Edited from DLsite]