This is endearment. It means more like a lover, like before they were married. Calling a girlfriend, wife etc 妹妹 is also acceptable. This is due to cultural preference where intimate public affections between man and woman is frown upon. So by calling someone, "elder brother" makes them look like family, or related to avoid people frowning upon such relationships. Incest is a huge NO-NO in Chinese society. Usually the male is older. Having a woman older is rare but still acceptable. For example, Empress Wu, the only female emperor in Chinese history is older than her husband, the Emperor by 6 years.

I think most of Korean do that, Chinese usually don't call their husbands "big brother". But if you ever heard that, that's just the wife being cute, like guys calling their wives "sweety".

Dear Cybelle, 你好！This culture originate from ancient China. When couple or man and woman didn't married,they want to go outside together,it's better to call each other as brother and sister, or people will make irresponsible remarks . As time passes，even they married ,still call each other as brother and sister. But now it's almost like a nickname. Some people are shy to say "my husband",they will say name+ge,as a nickname. Xie na call his husband as "jie2 ge"

No worries mate.

Oh I got it now,.. Great thank to you Justin,,,