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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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2014-07-24T15:36:41.090
17940
2014-07-24T16:50:49.670
3568
question
terminology
What do you call brothers or sisters of the same age?
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".
CC BY-SA 3.0
2014-07-24T15:52:28.883
17941
2014-07-24T15:52:28.883
17940
answer