My fellow weebsi found this out alittle bit ago and now i dont know what to say. Yoonbum is older but he never really knew who his mother and we are told that mother was out his life quite earlier on. So is there a possibility that she left bum for some reason and then got remarried to sangwoo dad and then had sangwoo. Thus making sangwoo and yoonbum half brothers.
and also you can have the free points if you want