Suzune Kyouka: Rezu Kaikin ~Gishi no Seidorei ni Sareta Watashi~.

Kyouka moves to Tokyo to prepare for her university entrance exams, staying at the apartment of her older brother and his wife. Though outwardly reserved, her mind swims with lascivious thoughts, leaving her with few female friends. Her sister-in-law Hikaru proves to be a woman of warm, uninhibited temperament who welcomes Kyouka naturally into their home. Kyouka finds herself gradually drawn into Hikaru's orbit, unable to resist her pull. While her brother is away on business, Hikaru ignites Kyouka's dormant eroticism and subjugates her as a sex slave, inexorably pulling her into an abyss of perversion.