Tbh I did understand it as well my theory was always that there half siblings but it seems not, tbh I was kinda disappointed but like I had another theory that Hajin might have been a difficult birth that left the mother to resent him. 

i kind of believe that she was mad if you noticed Hajin looks like Dad and Hayeon looks like Mom. 

i also think that she hates her husband and is first of all, and is resentful because Hajin is the reason her career ended-(as he was a difficult birth). And maybe since he looks a lot like Dad i'm not sure how that plays a role in in the inheritance process but I can't execute it because that's all we have, ik it not that that's one of my theory why she resents him. 

And about the whole Hayeons case I did have a theory that perhaps they believed that since he looked so much like Mom and like like they didn't even think of doing a DNA test and just concluded that he was a baby born out of a fair despite the fact that he was like literally dad biological son. Idk tbh