imouto 妹

The word imouto means "younger sister" in Japanese. Sometimes it's translated as "little sister" instead, but it's "younger sister." Even if your younger sister is bigger than you she's still your imouto.

Kanji

The word imouto written with kanji is imouto 妹. It's just a single kanji with a rather long reading.

Originally, the word was imo-hito いもひと, but through u-onbin ウ音便 it became imouto いもうと.^{[妹 - gogen-allguide.com, accessed 2019-11-08.]}

Imouto vs. Oneesan

The difference between imouto and oneesan is that imouto always refers to one's younger sister, while oneesan, oneechan, oneesama usually refers to one's "older sister."

So, in a sense, the opposite of oneesan is imouto, age-wise. These words only apply to female siblings, the opposite gender-wise, the words for "older brother" and "younger brother," would be oniisan and otouto.

Note that you can have multiple imouto's, multiple "younger sisters." Because the word imouto doesn't mean "youngest," just "younger."

The "youngest sister" of all your siblings is your matsumai 末妹, written with the kanji for "final," matsu 末, probably because she was the last one to be born. You can also simply describe her as ichiban shita no imouto 一番下の妹, literally "the imouto most below (in the age ranking.)"

Imoutosan

You may have already noticed that a lot of family members words in Japanese follow an o__san お〇〇さん pattern, but imouto does not. Why is that? Why is it that oneesan can end in san, chan and sama but imouto can't be imoutosan 妹さん?

This happens because this san stuff are called honorifics, and honorifics are used when you want to make a reverence.

Since your older sister is your elder, and you should treat your elders with respect, the term oneesan gets the reverencing honorifics. But your younger sister is not your elder, she's your junior, so you don't really make a reverence toward her.

One exception is when you're talking about other people's younger sisters. Since it's other people, you might want to speak with respect, so you say imoutosan 妹さん, with a san.

Since you normally don't use honorifics when talking about your own family to other people, imoutosan normally refers to someone else's younger sister, not yours.

Beyond imoutosan, there's also the word goreimaisama ご令妹様, which is an even more polite way to refer to other people's younger sisters, and would be used more in writing.

There's also imoutogimi 妹君, which is old-fashioned, but works just like imoutosan.

Manga: Houshin Engi 封神演義 (Chapter 2, 最初の封神)

Dakki no imouto-gimi...
妲己の妹君･･･
Dakki's younger sister...

Imouto vs. Imōto

The difference between imouto and imōto, with a macron, is merely that it's a different romaji for the same word.

This happens because long vowels, found in syllables like oo おお, ou おう, and oo おー, may be romanized with a macron instead of two vowel letters.