妹 mèi younger sister
Made up of [女 nǚ female; woman radical 38, 未 wèi not yet; did not; have not ]Made with 8 strokes.
Combines the female radical with a component meaning 'not yet' and so still a girl or younger sister - also serves as rough phonetic
Ancient small seal form 
Related characters
Also uses 女 nǚ component: 矮 ǎi (low) 安 ān (peace) 妇 fù (woman) 姑 gū (paternal aunt) 好 hǎo (good) 婚 hūn (to marry) 姐 jiě (elder sister) 楼 lóu (multi-story building) 妈 mā (mother) 奶 nǎi (milk) 如 rú (as) 始 shǐ (to begin) 她 tā (she) 妄 wàng (absurd) 姓 xìng (surname) 要 yào (want)