In Taiwan, 姊 (pronounced jiě) is the standard word for 'older sister,' while mainland China uses 姐 almost exclusively. 姊 survives mainly in the compound 姊妹 (sisters) in mainland usage.
grammar
Usually appears in compounds: 姊姊 (older sister), 姊妹 (sisters), 姊弟 (older sister and younger brother). Rarely used alone.
