HSK C1/W2/V1
Correct

妹

/ mèi / 
Strokes

Definition

younger sister

Example

As a Noun
younger sister
1
Xiōngmèi
兄妹

brother and sister

2
Xiǎomèi
小妹

baby sister; youngest sister

3
Èr  
二妹

second oldest of the younger sisters; next oldest of the younger sisters

4
Dàmèi
大妹

oldest of the younger sisters

As a Noun
address to a woman who is younger than the speaker and of the same generation
As a Noun
young woman or girl