。
She's my older sister.
Literal
She [topic-は] my older-sister is.
The bare 姉 (without the お~さん honorifics that would attach when speaking about someone else's sister, or addressing one's own) is the standard form for referring to one's own older sister to an outsider. Japanese splits 'sister' into 姉 (older) and 妹 (younger) — the same hierarchical distinction governs 兄/弟 for brothers — making relative age inseparable from sibling identity in Japanese. Relative age remains socially weighty: in-family hierarchy traditionally tracks birth order, and even casual addresses (お兄ちゃん, お姉さま) embed the older-younger axis.