Is oniichan also used to refer to a big sister?


Someone just told me their mother speaks Japanese and she says oniichan
doesn’t just mean big brother but also big sister. I thought oneechan is the
word for big sister though. He also said it’s really common in Japan to use it
that way, is that true?




No, that's not true. お兄ちゃん (oniichan) never refers to a female person.

But in rough slang, it can sometimes refer to a young **male** person in
general, just like some English speakers use "brother" or "bro" to refer to a
stranger. Maybe he has remembered this fact incorrectly? Likewise, お姉ちゃん
(oneechan) is sometimes used to address a young female person in general. See:
[How impolite is it to call a waitress
お姉{ねえ}さん?](https://japanese.stackexchange.com/q/17886/5010)

