
This is not enough for me, I wanted them to live together, get married and had 100 babies. I want more!!! ヽ(`Д´)ノ

Well as far as I can understand the translation would've been done way before the English licensing of the manga.. and since karenai hana has been licensed there is rare possibility that the manga would be uploaded here because there are so many mangas left half translated because it got licensed midway, so the translators stop working for it.

Lol, I don't think Oppa is that old, he might be only 10 year older from the guy, which is not that old. XD
(▰˘◡˘▰)

What do you mean by that? I don't get it. Anyway, here's the page I'm referring to. I don't understand korean but it seems to me that they are finding out each other's birthyear.
(spoilers, obviously)
https://toonkor.com/data/file/wtoon/15150281462604.jpg

Siblings with a 20+ years gap exist and sure, this may be more or less common depending on where you come from but not unrealistic at all.
Here's another link https://i.imgur.com/bffSoi4.jpg
Don should just stay with Nami and have normal life.
But he will be unhappy
Sure. He had to give up college and work to support his gifted brother and ailing mom. He gave up sports...he could've been a pro athlete or coach but he had to give up that dream. He was dying inside. Marrying her out of obligation for her support, having kids for her sake are the things that will finally hammer the nails in his coffin. Buried alive. Both Nami and him deserve better.
Yeah it's easier that way, but it could be bad for the longterm.