From the start of the show, we know that Casey is pretty much “obsessed” with Derek, and that even if she will never admit it, she value his opinion above all others. It’s like the minute her mother married George, her world decided that its new orbit would be Derek. It’s Life wih Derek. Not life with George and Derek and Edwin and Marti.
But Derek. I feel like Derek didn’t even try to hide his feelings for her half the time. Casey denies everything and sometimes you can really believe it. But Derek ? Derek makes her life “hell” but at the same time he always stands up for her, helps her when she really needs help, apologizes when he realizes that he’s been too far, gives her things, acts jealous the second she shows interest in an other guy. He keeps videos of her in the bathroom ! Many times in the show he looks at her like he can’t realize that this girl is so hot and brilliant and… damn, she’s his step-sister and he can’t show that he is completely in love with her.
Well, too bad Derek, because you ARE really that obvious.