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?
1 Answer
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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4Supported 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 Commented Jul 24, 2014 at 16:02
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1Twins 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 ♦Commented Jul 24, 2014 at 18:29
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3What if the order of birth is unknown?– user71Commented Jul 24, 2014 at 22:47
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