妹いもうと分ぶん

noun

sworn younger sister; protegee treated as a younger sister

Refers to a younger female whom one mentors or protects and treats like a younger sister, often in hierarchical or traditional settings (e.g., geisha houses, yakuza, or close-knit groups).

• 彼かの女じょは私わたしの妹いもうと分ぶんで、何なにかと面めん倒どうを見みている。

  She is my sworn younger sister, and I look after her in various ways.

• 先せん輩ぱいは新しん入いりの彼かの女じょを妹いもうと分ぶんとしてかわいがっている。

  The senior member treats the newcomer as a younger sister and dotes on her.