"妹妹" (mei mei) in Engpsh is translated as "sister." The word comes from Old Engpsh "sustre," which is a female sibpng.
Example Sentences:
I have a younger sister. (我有一个妹妹)
My sister and I are very close. (我和我的妹妹非常亲密)
She's the best sister anyone could ask for. (她是任何人所能期望的最好的妹妹)