Question

her younger sister wasn't in yesterday这个句子中，意思是她妹妹昨天不在家，为什么没有home？？？

Answer 1

这句话中的in是副词。

home用作副词的基本意思是“在家,回家,到家”,多用在be home, go home......

如：

Is he home from work?他下班回家了吗？

如果当名词，可用at, at home.

如：I'll be at home all morning. 我一上午都在家。《牛津词典》

Answer 2

这个句子要跟上文一起对照来看：
She was in home when her parents come home. But her young sister wasn’t in yesterday.