Is oniichan also used to refer to a big sister?

Question

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?

Answer 1

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?