second daughter
Refers to the second-born daughter in a family. Neutral, factual term.
His second daughter will be a university student next year.
The everyday counter for people: 一人 (ひとり), 二人 (ふたり), 三人 (さんにん).
More formal than 人 — used for headcounts in reservations, staff lists, and announcements (5名様).
Compound of 二 (ni, 'two') and 女 (jo, 'daughter; female'). The reading にじょ follows standard on'yomi.