Mack and the boys become a family. They live together in a place they call home, The Palace Flophouse. They make a plan together to go get frogs for Doc. They take care of each other and they all take care of the dog they have named Darling. They each take turns taking care of Darling when Darling gets sick. They exhibit behaviors inherent in families. They help each other. They can depend on each other. They go on trips together. The lesson Steinbeck is trying to reach about various ways families can be created, outside of biological combinations. A family is made up of people who love each other and help each other when they are needed. A family is what you make it. It doesn't matter if you are related, as long as you care for one another. A family can be made up of whomever you want. As long as you take care and help each other.