Responses
Spoiler
It's the og brother. I don't know how he does it all I know is that he does something and ends up bringing back his brother but as a child. Unfortunately I'm not entirely sure if the og brother has his memories. I haven't gotten that far into the novel all I know is that the 4th child is the og brother.

Can someone please spoil me (づ ̄ ³ ̄)づ
-How many "kids" are they going to have? From what I've seen in some comments there'll be more than 1? will they have them all at the same time like twins or triplets?
-How did they let the others know, especially Yoojin's brother? and how did everyone reacted?