Imouto Kansatsu Nikki

Main character discovers a hole in the wall leading to his sister's room. What he sees there is beyond his imagination - sister reads erotic books and gets unconsciously excited! Moreover, the erotic book is one from protagonist's own hidden stash. He can't hide his voice in exclamation, and his sister walks out to return him his book. From that day siblings relations can never be the same.

[From vndbreview]