Adult PC Game: Shoubo Seika -Haitoku no Majiwari- by Rose Crown Archive

Yuu lives with his mother, Seika. One day, twin siblings, Akane and Masaki, transfer to his class. Since they are lively and cheerful, they soon get popular at their class. Days later, Akane and Masaki talk to Yuu and tell him that they actually knew Yuu and Seika before they transferred to this school because the company Seika works is run by their mother. They invite Yuu to the office and he visits there without telling Seika about it.... It is a high-class brothel... where Seika works....