Setsunaku motomete: OL-hen (2000)

Kana, an office worker, lives with her mother, Sanoko, and her stepbrother, Shuji. Having endured an unstable childhood due to her mother’s free-spirited nature, Kana had found a sense of security in Ryuichi—Sanoko’s second husband—and enjoyed a peaceful life thereafter. However, following Ryuichi’s death, Sanoko’s life began to unravel once more. Seeking a replacement for the stability Ryuichi had provided, Kana enters into an illicit affair with Kurahashi, a business associate. Meanwhile, Shuji, while sympathizing with Kana’s plight, realizes that his own feelings for her have grown beyond mere sibling affection...