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eldest daughter; first-born daughter
彼女は三人姉妹の長女として生まれた。
She was born as the eldest of three sisters.
長女は来月大学を卒業する予定だ。
My eldest daughter is scheduled to graduate from university next month.
長女らしくしっかりした性格をしている。
She has a reliable personality befitting an eldest daughter.