How does Grampa's death affect the relationship between the Joads and the Wilsons?

Despite his tough veneer of anger and bitterness, Grampa dies from a stroke. Since he was the one family member most adamantly opposed to leaving their home, it was likely the separation that hastened his demise. In his parting words for Grampa Joad, Casy does reiterate his belief that people are the source of holiness.

The agreement between the Joads and the Wilsons to aid each other on the way to California is a significant plot development, for it is in collective interests that these families find their strength. This is the first building block in a collectivist scheme that Steinbeck seems to support in which working class people come together for their collective interests.