H~H

Spoiler



He realized after Yvonne stabbed / attacked him. I don't remember if it happened because he tried to protect his dad or Penelope or when Yvonne went "Well everyone found I'm not Yvonne so might as well kill you all~"

It takes him a while to come on terms that Yvonne is bad since she had a stronger hold on his emotions and the brainwash is effective on him because it feeds on his strong feeling of guilt since he lost Yvonne when they were young and the one that suggested they check the festival unsupervised. There's a guilt and delusion that he must take Yvonne side no matter what and give her all that she lost ( what Penelope end up getting instead) after her disappearance to make amends and fix what he has done.
He started to have doubts about Yvonne after this poison event because the investigation lead to Yvonne's maid providing the poison and he remembers his interaction with her very blurry and not understand why he acted a certain way.

I forgot to say he's susceptible to the brainwash bc of that the Prince and the pink haired brother aren't because the Prince has no relation to Yvonne, no emotion, sympathy or space in his hear for Yvonne so nothing could feed the brainwash. Pink step bro fell for the brainwash bc he was her brother but because he also started to see Penelope as his sister the brainwash broke since based on such emotion it didn't make sense why he would attack Penelope when he made a firm decision to be a better brother for her and fix the kain he has caused when they were young. Like such feeling in favor of Penelope is stronger than his feelings to be a brother for Yvonne. He loves his sister but he felt more in debt towards Penelope bc he was the cause of years of bullying she experienced.