妹夫 refers to the husband of one's younger sister.
Topic 1.1: 1.1 Families and Communities in China
妹夫 (mèifū) cheat sheet for homework
The husband of one's older sister.
兄弟姐妹 (xiōngdì jiěmèi): Siblings; brothers and sisters.
夫妻 (fūqī): Husband and wife.