Derek was king of the castle. He always had been. His dad was too nice, and his brother too easily intimidated, to ever challenge him on that. (Marti may have been an issue had she been a little older, but Derek figured that by the time she was big enough to give him some true competition, he'd already be out of the house.)
When his dad started getting serious about Nora and started hinting about marriage and about two new sisters, Derek didn't think they'd be a problem. Nora wouldn't do anything because she was sensitive about not upsetting the balance of their house even more than they had to, and then when he met Lizzie, like with Marti, he thought there could be some backbone there, but in the end, she was too polite to take him to the mat.
But for all the signs, he didn't peg Casey as trouble. A keener? Yeah, that was practically stamped on her forehead. A killjoy? Most certainly. But he figured she'd be polite like Lizzie and easy to bowl over.
Derek didn't like to admit when he was wrong, but WOW, was he wrong. Not only was she just as used to getting her own way as Derek was, but she would not back down. Like, at all. And just when he thought he had her figured, she goes and manipulates him into giving her exactly what she wants! Who does that? No one outfoxes Derek Venturi.
Luckily, Nora stepped in and called her on it. But Derek got a taste of what he was up against. There was a challenger for his throne. And one that he couldn't scare off, bribe, or charm.
He had a feeling, life was about to get interesting.