Familiar, affectionate term for one's own older sister or a close older female friend. Often used by children or within the family.
See also: お姉さん
Big sis, let's play together!
Informal, friendly way to address or refer to a young woman, often by a child or in casual settings. Can sound slightly old-fashioned or dialectal.
My older sister is a college student.
Hey, young lady over there, could you give me a hand?
From 姉 (ane, 'older sister') with honorific prefix お and familiar suffix ちゃん.