This is the 2422nd most frequent Chinese word.
妹妹
Younger sister.
Here, "妹妹" is used as a noun to refer to a younger sister, indicating familial relationship.
我的妹妹很喜欢看书。
My younger sister really enjoys reading books.
In this sentence, "妹妹" is used metaphorically to describe someone with a caring and close relationship akin to that of a younger sister.
她像我的妹妹一样照顾我。
She cares for me just like a younger sister would.
Here, "妹妹" is used in possessive form to indicate that the subject 'school' is related to my younger sister.