how important is Ma to the family ?

Ma Joad is a woman accustomed to hardship and deprivation. She is a forceful woman who is determined to keep her family together at nearly all costs, yet remains kind toward all, even sparing what little the family has for those even less fortunate. Ma is the glue that keeps this family together despite the worst trials and tribulations. In chapter 13, Steinbeck sums up her role in the book,

"She walked for the family and held her head straight for the family."