Bishounen no Geboku (?) ni Narimashita

Haruhara Kana, our heroine, was living a picture-perfect, ordinary, and relatively peaceful life. However, one day, she accidentally breaks something belonging to a pretty-faced underclassman.
The pretty boy says that she does not have to pay for the damage, but when she pleads with him that she still wants to make up for it to him, he acquiesces: "Alright. Please be my servant.''
　Why did he request something like that from her!?

　And what will happen when her siscon of an older brother finds out about this absurd situation she found herself in!?