姐妹(jiě mèi) - sisters, female siblings in Mandarin

姐妹 (jiěmèi) is the collective word for sisters. Chinese normally insists on marking which sister is older, so 姐姐 and 妹妹 are separate words, and 姐妹 is what you reach for when you want to talk about them together without ranking them.

Counting works differently from English here. 三姐妹 means three sisters in total, not one girl plus three siblings, so a family of two daughters is 两姐妹. The pattern 姐妹俩 (the two of them, sisters) is very common in speech. Beyond the family, 姐妹 does duty for close female friendship and for institutional pairings: 姐妹城市 are twin towns and 姐妹学校 are partner schools.

The trap is grammatical rather than semantic. 姐妹 is a group noun, so you cannot point at one person and call her your 姐妹 in ordinary conversation; that needs 姐姐 or 妹妹. And note that the parallel word for brothers, 兄弟 (xiōngdì), does slide into the singular as a term for a mate. 姐妹 does not follow it.