
They share a cousin, have the exact same looks but different colour scheme, and call each other brother. Even without a third party, its pretty clear they're meant to be twins, or at the very least very similar appearance relatives.
It seems pretty clear that the twin did it, he clearly has a deep hatred of Nyeonbi and disapproval of Gae Won's affections for her. He looks the same as Gae Won to be mistaken for him, is his relative so knows him well enough to impersonate him, and hates Nyeonbi so he has a motive.
Him pretending to be Gae Won, tricking Nyeonbi into drinking the poison and then leaving Gae Won to find her dying/dead to assume she took her own life. Its a much more plausible explanation than "Gae Won was forced to give her the poison"

At chapter 35, there was a guy dressed in red+white clothes named "Unho" called Wongae as brother.
And he told Wongae to visit "their" cousin Sisi in the Palace (which Wongae did visit on chapter 36).
He looks like the "bro-con" type who idealised his brother as this some kind of "hero".
And so, a "hero" shall not have a relationship with a "villainess" (Nyeon Bi).
That's why I think this Unho guy is the culprit behind the whole "betrayal" things.

Now that I’m paying attention they really do look alike your theory is possible however they got diff hair colors that’s why my brain didn’t recognize they looked similar but they might also just be cousins because i think in Chinese culture ( I’m Chinese) you refer to your cousins as brother or sister if your close or something I don’t remember but now that I see it that twin theory is actually quite possible
i'm so confused about won gae. he did kill her right? or was it an imposter? why does he seem to genuinely love her?