小姨子

xiǎoyízi
(coll.) wife's younger sister; sister-in-law (term not used to directly address her)

CEDICT

colloquial

Entry Composition

1 小姨 xiǎoyí
  1. mother's youngest sister
  2. wife's younger sister; sister-in-law
2 子 zi noun suffix, as in 椅子 yǐzi "chair"