Imouto is the Japanese term for little sister, used for actual blood relatives, or people standing very close to one, and is like a little sister.
In these cases, unless given consent, imouto-san or imouto-chan is the decent phrase, fitted with the necessary honorific, giving respect.
The big brother version of the phrase, onii-chan is also utilized widely.
Spread and Usage
Thanks to message boards, forums and blogs revolving around anime and manga, the phrase got adopted in plenty of peoples’ dictionaries.
Urban Dictionary added a definition on imouto in 2009, to enlighten all to-be otakus.