Whahahahah ikr! By the cameo story how the chairman couldn't move on after the other guy death-i forgot which chapter and since then, their whole relationship become sus lol-no, actually it's just my Fujo brain is working there.
I even think that sera is Mr baek's daughter but not through infidelity but like sperm donation lmao but my theory just collapsed in ch 55 when seeing Mr baek mentioned 'brother'
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Company: Seongwon Group
Owner: President Shin
Adoptive daughter: Shin Se-ra
Wife: Ms. Song (Shin Se-ra and Han Jia's biological mother)
Director: Baek Ji-hwan ((Shin Serah and Han Jia's uncle)
Older brother - Unnamed (Shin Serah and Han Jia's biological father - deceased)
Protagonists:
Shin Se-Ra (Sarah) - twin sister's Han-Jia
Han-Jia (Jiah) - twin sister's Shin Se-Ra - MC
Ally:
Lee Ro-Woon - ML
The group originally had two co-chairmen. Chairman Shin is the father of our FL (not really cause of the mom cheating issue) and the other Chairman was Eeron's father.
The other Chairman died off screen maybe 10+ years before the story starts and Eeron was taken in as a ward (his mother hid him before his father's death because she's just as twisted in love as Eeron and didn't want to lose the only token of her love to a man who never came back). Also, Chairman Shin is gay and was in love with his co-chairman (so take what you want from that and how that may inform your understanding of the mom's paranoia and gradual mental decline).

I think Chairman Shin is gay and has a thing with Eeron's (or whatev his name is so confusing) father. But yeah to keep with the appearances they had to pretend that Sera is his daughter....IDK JUST MY ASSUMPTIONS