Two teenagers living together in one house is hard enough, especially when you're not getting along with each other at all like the two oldest siblings of two families, Derek Venturi and Casey McDonald. Casey was forced to merge into a complete stranger's house in London, Ontario, which is the Venturi's home, from her own home-city in Toronto, Ontario, after their parent's engagement. While the other siblings are getting along with one another just fine, Derek and Casey are in the exception, whom don't get along at all, as you can see in the first and second seasons, actually in the earlier days of their parent's marriage. However, in few episodes, it was shown that they actually don't dislike or hate each other. Deep down, they do care about each other and help one another out with the problems they have of their own, but they just don't show it. Their relationship starts to develop as siblings and sometimes as friends/true friends. Derek is starting to have a more brother/sister relationship with Casey, and Casey starts to think of Derek as her brother, as said on the finale episode of Season 4.
Though they always tries not to show that they really care for each other, it just comes out by itself; And through that, they become closer, though they still are bickering with each other, but they tend to work things out better than their earlier days. Sometimes words doesn't need to be said that they care, actions shows it themselves.
I understand why many people ship Derek and Casey together as a couple because they look cute together. In addition to that, the reason that they are step-siblings, therefore, it's legal for them to become more than just siblings. However, if you look throughout the whole series, you may feel the tensions between them are actually more than what siblings would do, but I can see, they try not to pass over that line. Sometimes, despite all of that, I can also see that they actually develop their relationship as a family-love though. In my opinion, I think Casey and Derek are better off as siblings, it makes their relationship stronger and longer as that than as a boyfriend/girlfriend relationship. I don't want to see any of them feeling hurt and awkward around each other after their break up, and besides, they are living in the same roof and they have to see each other every day and night, how can they bare see each other again if that happens? Them, as siblings, makes them develop more stronger relationship with each other than a boyfriend/girlfriend relationship. So I love seeing Derek and Casey as a brother and sister than a boyfriend and girlfriend.