Several parts combine into one character.

妹 is a phono-semantic character: 女 (woman) carries the meaning, 未 (not yet) lends the sound. 未 was originally the young, not-yet-grown branch at a tree's tip — so "a woman not yet fully grown," that is, a younger sister. Sound and sense blend here.

Korean hanja-sound "mae": 男妹 (nammae, brother and sister), 姉妹 (jamae, sisters), 妹弟 (maeje, a younger sister's husband), 令妹 (yeongmae, an honorific for another's younger sister). In 兄弟姉妹 (siblings), 妹 is the youngest rank.

Mandarin: mèi, 4th tone. 妹妹 (mèimei, younger sister), 姐妹 (jiěmèi, sisters), 表妹 (biǎomèi, younger female cousin), 小妹 (xiǎomèi, youngest sister).

Japanese: on-reading マイ (mai), kun いもうと (imōto). 妹 (imōto, younger sister), 姉妹 (shimai, sisters), 義妹 (gimai, sister-in-law / stepsister). The native word いもうと derives from imo (a younger female) plus hito (person).

It pairs with 弟 (younger brother) seen earlier: 弟 (younger brother) and 妹 (younger sister) divide the junior siblings.

Memory aid: woman (女) plus not-yet-grown (未) — a younger sister not yet fully grown.