Responses
I think more than replacement, she wanted to create a perfect son which her sickly son could never become. She wanted to project her own desire to be a successful model in seo in and as seo in was looking for approval, he grabbed the opportunity too.
But I just cannot simply dismiss that all these means she didn’t love him. Just like her husband she could cut ties with him too. But she didn’t despite seo in causing "trouble" for her on numerous occasions.
I mean, hope this kind of parental love finds no one but I feel it’s presence(as an asian child lmao)

The whole thing started because both seo in and yw had fucked up parents.
I love how all those pure evil characters(seo in's brother, yw's father) are dead.
But is it too far fetched to think seo in's mother is still salvageable? Though what she did is unforgivable, I feel that she loves SI in her own fucked up way and wishes him well. She needs some serious therapy to channel her love in healthy way tho. I wish the mother and son have an honest talk and she apologises to both yw and si because it seems she has remorse.