‘姨妹’ (yí mèi) is a kinship term meaning ‘younger sister of one’s mother’ — that is, the younger biological sister of your maternal aunt. The first character 姨 (yí) refers specifically to one’s mother’s sister (aunt), while the second character 妹 (mèi) means ‘younger sister’. Together, they denote a precise familial relationship: not just any cousin, but specifically the daughter of your maternal aunt who is younger than that aunt. This term reflects Chinese kinship’s emphasis on distinguishing relatives by generation, lineage (maternal vs. paternal), and relative age.

‘姨妹’ is used formally and respectfully in speech and writing, especially when introducing or referring to family members in contexts like weddings, family gatherings, or official documents. It is not used for cousins on the father’s side (those are 表妹 biǎo mèi) nor for older sisters of one’s mother (who would be 姨姐 yí jiě). In daily conversation, some speakers may simplify it to ‘小姨’ (xiǎo yí) in informal settings — but this is technically inaccurate and context-dependent.