I think it was because he was less inclined to believe her in the first place.
From what I remember, duke and Reynold wasn't successfully brainwashed because they hold platonic affections for her, and sees her as a person.
I think it was because he didn’t want to become real sibling/family with Penelope?
Derrick actually in love with Penelope and that’s why he bullied her and refused to accept her as sibling/family.
While Reynold and the Father really want to accept her as family, Derrick would never do. Maybe that’s why Yvonne’s spell works much more on him?
im quite confused on how derrick still under yvonne spell? bcs reynold and her father so so different with him eventho he also under the spell. why derrick such a shit. ughhhhh