This has had me curious for a long time now. In Japanese, I know there's no word for "brother" or "sister" without specifying older or younger (兄、弟、姉、妹） but what happens if the person's a twin? This also goes for 伯父さん　叔父さん　叔母さん and 伯母さん)

I dont know the kanji, but the word for twin is ?futago?

Maybe this?

双子

I have a twin brother, I usually just say 弟　since I'm five minutes older,　unless I'm actually talking about having a twin in which case I specify 双子　or 双子の弟

I have heard few other pairs of twins refer to each other by who was the first to come out and who was the second. I have never heard someone refer to their twin neutrally so far.