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From what I can see people are forced to use the term of a younger or older brother/sister oneesan, oniisan, etc.

But what do you call a brother or sister if they're the same age as you?

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up vote 7 down vote accepted

I think that even twins, born only moments apart, are considered to be older and younger based on who was born first. So even if you're both twenty years old, the normal rules apply.

That's why you can say things like 双子のお兄さん "older twin brother".

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Supported by this 知恵袋 answer, which also has some interesting information: Historically, it was actually considered that the twin born later would be the one considered the older brother / sister! This was because they were deeper in the womb, implying that they "were there first". But nowadays, yes, the twin born first is considered the older sibling. detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1414437299 – Aujury Jul 24 '14 at 16:02
Wow that's interesting. Talk about strict semantics! – Tek Jul 24 '14 at 17:27
Twins are entered in the first-born first order in the 戸籍謄本 as well and the first-born twin is considered 長男 (in the case of two twin boys with at most older sisters). – Earthliŋ Jul 24 '14 at 18:29
What if the order of birth is unknown? – rightfold Jul 24 '14 at 22:47
@rightfold Sounds like an additional question to ask :3 – Tek Sep 25 '14 at 14:52

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