The Chinese word for step-mother is
继母

jì mǔ

Chinese Definition

step-mother
1. 阿姨
Components
继
1. Simplified form of 繼
母
1. mother
父母 - father and mother
母子 - mother and son
Antonyms: 父
2. (Used to address a female elder member of a family.)
姑母 - father's sister
祖母 - grandmother
Antonyms: 父
3. (of animal) female
母牛 - female cow
Synonyms: 雌
Antonyms: 公
4. pertaining to origin
母校 - alma mater