eldest sister
Rare term for one's eldest sister. In modern Japanese, 長女 (eldest daughter) or 一番上の姉 (oldest sister) are far more common.
In old documents, 大姉 is sometimes read as おおあね and refers to the eldest sister.
The kanji 大 (big/great) and 姉 (older sister) combine to literally mean 'great older sister'. The exact historical derivation is uncertain; it is a rare, possibly archaic formation.