First off, the difference is actually very simple. The words otouto 弟 and imouto 妹 are always used for the younger brother and sister respectively.
So, say you have two sons. One is younger than the other, so he's the otouto, the other, obviously, must be older, so he's the oniisan お兄さん. Likewise, if you have two daughters, the younger one is the imouto 妹 and the older one is the oneesan お姉さん.
Oniichan, Oniisan, Oniisama
You might have noticed that some characters call their older brothers oniichan, others call them oniisan, and others oniisama. But wait a second, what's the difference between oniichan, oniisan and oniisama?
As you probably guessed, this is actually related to the Japanese honorifics kun 君, chan ちゃん, san さん and sama 様, except there is no oniikun. When someone says oniichan, the chan there implies intimacy. And when they say oniisama, the sama there implies the utmost respect. By the way, oniisan is neutral and implies nothing.
The same is true for oneechan, oneesan and oneesama.
As if that wasn't enough the o is actually a prefix and is optional. Actually, even the san is optional. That means all of the following words mean "older brother" in Japanese:
- nii 兄
- niisan 兄さん, niichan 兄ちゃん, niisama 兄様
- onii お兄
- oniisan お兄さん, oniichan お兄ちゃん, oniisama お兄様
And for "older sister":
- nee 姉
- neesan 姉さん, neechan 姉ちゃん, neesama 姉様
- onee お姉
- oneesan お姉さん, oneechan お姉ちゃん, oneesama お姉様
Other Words for Brother and Sister
Besides the words above, you can say older brother and older sister in Japanese the following ways:
- ani 兄 and ane 姉
- aniue 兄上 and aneue 姉上
- aniki 兄貴 and aneki 姉貴
Brother and Sister
In the last case, aniki and aneki are also a way to call the "boss" of your gang, if you had one and if you were a character in an anime.
Brothers and Sisters
Behold the Japanese language! When you have two brothers, one must be the ani 兄, the other, the otouto 弟, together, they combine, to form... *drum roll* ...kyoudai 兄弟!!!
No, really. I'm serious.
The word for "brothers," two or more, in plural, is kyoudai and is written with the kanji for older brother and younger brother together. That's because a single kanji can have multiple readings.
As if that wasn't enough, you can actually join ane 姉 and imouto 妹 to form shimai 姉妹, which would be the word for "sisters."
But wait! There is more! Since there's a 50% chance the pair of siblings aren't the same gender, often, in manga and written Japanese, you get stuff like kyoudai 姉弟 or kyoudai 兄妹. It reads the same as the kyoudai 兄弟 from before, but the kanji written change to match the genders of the siblings.
(technically, the words should be read as keimai 兄妹 and shitei 姉弟, but nobody reads them like that)
Finally, kyoudai shimai 兄弟姉妹 is how you say "brothers and sisters" in Japanese.