We decided, hey, let's give Luke an older brother. And not just any older brother. One he looks up to as an idol, which a lot of siblings in this comic tend to lack. You look at Riley and Doug - they hate each other. Troy and Blake - Troy hates Blake due to him babying him all the time. Vincent and Spencer - they end up physically fighting, trying to kill each other. And those aren't even a lot of examples of siblings. There's all the angels as well, and clearly they don't get along well. But Luke and Holden will be that different set of siblings that work well together, and kind of survive with each other since their parents aren't mentioned, thus insinuating that they travel/are homeless.

/Riley