Family is a means of survival in The Grapes of Wrath. Without each other, the Joads would have no way of coping with the loss of their land or of getting to California. Family is the one weapon that the Joads have against the cold, bitter world around. They, along with many other migrant workers, learn that they are stronger and safer when they reach out to other families, when they create a sense of community.
Family is the only means of survival in the Dust Bowl world of The Grapes of Wrath.
There is no such thing as "family" in The Grapes of Wrath.