Common interests is a result of the shared experiences that these folks have known. There develops a pattern of concern that may or may not be spoken that works to give life or to sustain it for the participants in the family unit. Common goals are essential to the development and endurance of family. The family exists to do something. Those acts and goals are changing and not always fully defined, but they are real and essential parts of sustaining family.
Common order is another component of family.
This gathering has learned the system of relating to each other. They may not have written those customs down, but everyone in the family comes to understand the design of the system that they live in. They may not always like the rules of this system, but they know the consequences and choose to live with them so as to be an enduring unit. The family is thus in part the result of what is held in common by a gathering of people. But the family is much more than just a group struggling to hold things together.