姊 (zǐ) is a bound form meaning 'elder sister' — it almost never appears alone in modern Chinese. You will see it in formal or written compounds, especially 姊妹 (zǐmèi, 'sisters'), and occasionally in kinship terms or regional speech. Unlike the common word 姐妹 (jiěmèi), 姊妹 carries a slightly more literary or respectful tone.