What is the relationship between grandpa and grandma in Grapes of Wrath?

" Behind him hobbled Granma, who had survived only because she was as mean as

her husband. She had held her own with a shrill ferocious religiOSity that was as

lecherous and as savage as anything Grampa could offer. Once, after a meeting, while

she was still speaking in tongues, she fired both barrels of a shotgun at her husband,

ripping one of his buttocks nearly off, and after that he admired her and did not try to

torture her as children torture bugs. As she walked she hiked her Mother Hubbard up to

her knees, and she bleated her shrill terrible war cry: "Pu-raise Gawd fur vittory."

Granma and Grampa raced each other to get across the broad yard. They fought

over everything, and loved and needed the fighting. "

Basically, grandpa and grandma were both mean and loved to pick fights. They could take it out on each other, and loved to do so. In fact, they needed the fighting, probably to get out their aggression and keep them connected.