I love your question because it's not something that a reader typically thinks about while reading this novel. Your question, therefore, makes us all think outside of the box. The Outsiders, which is told from the viewpoint of Ponyboy, a disenfranchised orphan, is ironically telling the story of family, not only as biological members, but as a group of young men serving different roles in the family through gang membership.
Sixteen years on the streets and you can learn a lot. But all the wrong things, not the things you want to learn. Sixteen years on the streets and you see a lot. But all the wrong sights, not the things you want to see.
After the death of their parents, Ponyboy, Sodapop, and Darry are left to maintain a family. Being the oldest, Darry becomes a father figure to Ponyboy and takes on much of the responsibility for the needs of the family. Sodapop is a jokester, often bringing levity to situations.
Tragic circumstances ensue through which Ponyboy and his close friend Johnny find themselves running from trouble. During this time Ponyboy has realizations about the fleeting moments in lives that are of great importance. Ponyboy is proud of his background and social group, but he hopes to rise above the familial situation from which he came. Here are Pony's thoughts on the subject:
Suddenly it wasn't only a personal thing to me. I could picture hundreds and hundreds of boys living on the wrong sides of cities, boys with black eyes who jumped at their own shadows. Hundreds of boys who maybe watched sunsets and looked at stars and ached for something better. I could see boys going down under street lights because they were mean and tough and hated the world, and it was too late to tell them that there was still good in it, and they wouldn't believe you if you did.
Suddenly, Ponyboy thinks beyond his own personal tragedy and expands it to the possible tragedies of so many other young boys in gangs. Lots of young men living on the streets and instigating with violence. Ponyboy thinks hard about how, just maybe, some of them watched sunsets just like he does. Then Ponyboy considers how many of these young boys have died or "gone down" just like members of his own family and, especially, like Johnny. Ponyboy is overcome by these thoughts.
Although one can move within and across social status lines, the importance of the moments in which one considers family are paramount, and Ponyboy realizes the importance of his close friends as family.