I'm in love with my sister; abbreviated to BokuImo) is a manga series by Kotomi Aoki. Originally serialized in Shōjo Comic, the individual chapters were published.
Movie: My Sister, My Love; Romaji: Boku wa imouto ni koi wo suru; Japanese: 僕は Yori and his twin sister Iku used to be very close as children but lately Yori has them as step siblings so I'm just going to ignore the facts and be oblivious.
Diary of a Step-Sister is an otome game by Voltage. Your mother marries a wealthy man with five sons and you fall in love with one of your step-brothers.