I think Grandpa and Grandma were Ma's parents just by her relationship with them and the context of the family construct in that time period. The father left his family as seen through his own sons later on. American, presumably Protestant families in that time period were matriarchal, as you see through the weight of Ma's character through her strength being the basis for the actions of the book. She made the final decision to leave the boxcar, talks as the representative for the family in important decisions, a role she's only allowed Pa, and through most of the book, Tom, to make. My Protestant grandmother was a child when this book came out, and my family's dynamics are pretty similar to the Joad's. My parents live 10 minutes away from my mom's parents, while my dad's are much further away. This likely doesn't represent all families, but I'm pretty sure this is why Grandma and Grandpa live with the Joads. We know very little about Pa's parents nonetheless and if they lived with the Joads too.