妹 is a sound-and-meaning character already found in oracle-bone and bronze writing. 女, “woman”, gives the meaning category; 未 supplied the ancient sound clue. Early oracle-bone texts often borrowed 妹 for a different word connected with the dim time before dawn, while bronze inscriptions also use it for a younger sister. The component sketch avoids turning those two parts into a made-up picture story.
A piece of culture
Two kinds of sister English and Dutch can simply say “sister” or “zus”, but Chinese usually marks relative age: 姐 is an older sister and 妹 is a younger sister. Put them together as 姐妹 (jiěmèi / ze2 mui6) and the word can refer to sisters as a group. The pair makes family order visible in everyday language.