SPOILER:
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That was the poison and antidote (?) that Penelope asked from Vinter. In Penelope's debutante, Yvonne will be introduced even though she shouldn't be because of Derrick, and as Penelope will toast with Yvonne, the same type of poison she ordered from Vinter is mixed with Yvonne's drink and she realizes that scheme so Penelope drinks the poisoned drink herself with the intention to k-word herself.
spoiler
read it long time ago but long story short, real one appears on mc coming-of-age ceremony unplanned, mc drinks poisoned wine (don't remember circumstances), trying to exit the game. Almost died but being saved, further it goes the worse all the situation becomes
the real one manipulates other's minds, so the older brother being brainwashed, thinks mc did all of that to get attention. her 'dog' cooperates with the real one trying to make up with mc, haven't read further cause i can't
Here is where the shit hits the fan our poor baby