Ayaka

Ayaka is the oldest of the three sisters. She hates her older brother, Shintarou, however, once he transforms into a child, unbeknownst to her, she falls in love with him and has sex with him. Her sisters are Fuyuno and Chiaki, and eventually they start fighting over who gets to have sex with Kouhei/Shintarou. According to Fuyuon, she's a huge shotacon.