妹妹英语怎么写 ？

younger sister 小妹妹

sister 既有姐姐也有妹妹的意思，所以当表达妹妹要加上younger

双语例句

They are my older brother and younger sister.

他们是我的哥哥和妹妹。

I have one younger sister.

我有一个妹妹。