Osananajimi no Kanojo to Namaiki na Imouto o Bitch ni Shiteyatta Fuhaha (Game Version)

Two young girls go from sweet innocence to hardcore sluts. Starring imouto Misa and childhood friend Nodoka. "My name is Yuuichi, and I have a little sister. She's not just the smartest girl in her class, but the whole school. No matter what I compare poorly to her. My imouto -- Misa -- looks down on me now. If I catch her eye she'll spew abuse at me. Getting mistreated like that has built a black ball of misery inside me... Someday I'll have my revenge. Someday."