I can totally see this but also…. He kind of made that decision and he needs to suffer the consequences. He CHOSE to not see her as a sister, he CHOSE to fall in love with her, he CHOSE to act shitty and dismissive to her all this time. It was all actions he did without the influence of anyone else.
I know all that but still hate him he was indifferent to her suffering even before he have feelings for her she was a child who just want a family but she suffer all that because of his pettiness penelope died many times and many of those was his fault too im sure and even till now not one sorry is uttered from him he still denying and blaming her hes selfish af if he did all that just to hide his feeling for her
But derrick was a child too. A greiving child who lost his sister, and then another girl comes in to take his sister's place. The duke is more to blame in this situation, he should've helped the kids adjust properly, not just leave them to sort it out themselves. It created a lot of hate between the brothers and Penelope, and led to penelope's personality being very volatile, and yet I never see ppl hating on the duke much. Ppl started liking reynold when he started treating Penelope well, so I'm hoping for derrik to be given the same treatment if he gets a good redemption arc. ofc, the redemption arc must be done well, he has been a complete ass this whole time, that I won't deny

I honestly just feel bad for him. He never saw Penelope as his sister so he ended up loving her instead. He knows it's wrong tho, and in the middle of all that bull of his conflicts yvonne came back, so he tried to further supress his feelings by giving preference to yvonne. This made him way more susceptible to the brainwashing, and he ended up in this state. He's got his wake up smack now tho, so I'm rooting for his redemption arc