She's dating Ijun as KIM Siyeon, not CHOI Siyeon (real self and name). That means she chose who to marry as a dependant of Kim Donghan. KIM Siyeon would automatically be in line to marry Ijun instead of the brother. If she reveals it early, it could mean that Eunhye would replace her spot and be Ijun's fiancee.
That, and there's also a hidden info by the monk.
p.s. I honestly think the only bad mistake she did here is accepting Ijun's feelings early on in the story. If she didn't, she could've faced them head on. But I'm glad she did tho cause that lessened the drama and misunderstanding.
*descendant
Thanks for explaining!!
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Why FL acts dumb and slow, her character is so weak even after going back in past, I guess it's just me but FL gives me headache also why is she dragging things with Gwinye(or whatever his name is) when she is in love with other guy also the emotions shown by FL is so tasteless!!!