This quote shows how the Joad family is connected by love, and the foundation of that love in the Joad family was Granma and Granpa, and once they both die, the family becomes less stable. This is seen throughout the novel.
Responsibility -- be a Godly parent. Amen. I wish more parents would realize this. Trying to force your children to fit into a "perfect godly child" mold will only make them resent the you and the faith. Lead by example.