We readers aren't allowed much access into the world of Grandmother; the little we do know is through the context of the Lee family. We know that Grandmother and Mother run the family farm, but that Grandmother's old age makes it hard for her to help much (1.2). Grandmother is also one of the sources of demon tales, building up Moon Shadow's expectations of the wealth of America (1.16). She is against Moon Shadow leaving for America at first because she has lost all of the men in her life to the Land of the Golden Mountain. Grandmother also has good reason to distrust "demons": her husband was lynched by Americans, after all. We also know that Grandmother thinks it's idiotic and disgraceful for Windrider to dedicate any time and effort into the idea of flying.
The fate of Grandmother remains unknown; Father returns to bring Mother to America at the book's end, but Yep makes no mention of Grandmother. This might speak to a sense of history and generational change in families that transition from China to America. What happens to the people, places, and traditions of one land when a family migrates to a new one? What, or who, gets left behind?