what do I say? (TW for fat hatred)

My parents just returned yesterday from a week-and-then-some trip to Yellowstone with my maternal grandmother. I picked them up from the airport around 4:30 PM. A few initial pleasantries got us out of the airport, but once we hit the highway, the conversation turned to dinner.

Dad: "What do you want to eat for dinner, Mom?"

Grandmother: "I don't even...all we do is talk about food! Where do you want to go, what do you want, blah blah blah. All we did the whole time was eat and talk about food! I don't want to talk about food."

(Background: My mother and father are fat. I am fat. My grandmother is not. She is one of the very few people in family who are not fat. My mother, her sisters, my cousins - almost everyone except my grandmother is fat.)

My mother is getting that tense thing in the back of her neck. I am driving, and she's in the back seat, but I can feel it, because I do it, too.