What is the word 'sister' when translated from English to Japanese?

Oneesan is older sister.

Imoutosan is younger sister. Older sister is onee-chan and younger sister is imouto. Also, older brother is onii-chan and younger brother is otouto. Older sister - (when you're talking to her or talking about someone else's sister) is Onee-san.

If you're talking about them to someone else, then it's ane.

Younger sister - Imouto