In some ways they really were one big family. They fought and bickered, but they would always be there for each other when it mattered most.
George and Nora were scatter brained to the extreme, but they loved their kids. Somewhere down the line they had stopped being hers and his and become theirs. Casey and Lizzie were just as much his daughters as Marti was. Derek, Edwin, and Marti meant just as much to her as her own daughters did. In this case, the "Step" made no difference and they were a family.
Casey and Lizzie were Marti's big sisters as much as Derek and Edwin were her big brothers. Lizzie had become Derek's little sister and Edwin had become Casey's little brother. They all fought for supremacy as siblings did. They all protected each other as siblings did. They all loved each other as family did. The "Step" could be discarded.
Edwin and Lizzie weren't siblings; they got along too well as equals to be called that. Calling them brother and sister would cause no harm but it's not what they were. They were best friends and partners in crime. They loved each other platonically, but they weren't family. The spark they had that made them such great friends had the potential to turn into something more. It never did, and they weren't meant to be or anything like that; but the potential was there and they weren't family. In this instance, the "Step" mattered, but only a little bit.
Casey and Derek weren't siblings. There was nothing familial about the way they looked at each other. It could be argued that the reason they weren't siblings was that they met when they were teenagers. They could've been raised together since birth and they would never be "siblings". There was too much fire in their relationship to be put in such a platonic category as family. His protectiveness of her was not, in any way, brotherly, and her interest in his life was not as a sister. No one had ever doubted that they would go to the ends of the earth for each other, and it had become impossible to think of one without thinking of the other as well. The only way they would ever be "Family" is if they got married. The "Step" was the difference between something sick and something beautiful.
In the end, there's no such thing as a Step-family. Love doesn't adhere to words or social norms; love just is. The love between Edwin and Lizzie isn't romantic or familial but just love, plain and simple. The love between Derek and Casey is the love between soul-mates and is by definition not so simple, but it's real and it's strong and it's unconditional. They all loved each other, and maybe that's what really mattered; not the types of love, but that love was there at all.
In some ways they really were one big family.
One big, dysfunctional, royally screwed up family.
Is there any other kind?