Origin

This character combines the radical for "woman" or "female" (女) on the left with a sound hint (未) on the right. The 未 component doesn't describe what a younger sister does or is — it mainly tells you roughly how to pronounce the character. By pairing a female classifier with this sound hint, the character came to mean specifically a younger sister, distinguishing her from an older sister or other female relatives. The shape efficiently packages both the category (female family member) and a guide to its pronunciation into one character.