Jim's grandfather is distinguished by his religious nature. We don't get a lot of description here, so let's take a close look at the little text we have:
My grandfather said little. When he first came in he kissed me and spoke kindly to me, but he was not demonstrative. I felt at once his deliberateness and personal dignity, and was a little in awe of him. The thing one immediately noticed about him was his beautiful, crinkly, snow-white beard. I once heard a missionary say it was like the beard of an Arabian sheik. His bald crown only made it more impressive.
Grandfather's eyes were not at all like those of an old man; they were bright blue, and had a fresh, frosty sparkle. His teeth were white and regular--so sound that he had never been to a dentist in his life. He had a delicate skin, easily roughened by sun and wind. When he was a young man his hair and beard were red; his eyebrows were still coppery. (1.2.8-9)
Grandfather's very looks suggest a venerability and wisdom that is later confirmed by his words and actions. He is always reading prayers for the family, and the most notable thing he does in these early chapters is give a eulogy at Mr. Shimerda's funeral. In his infinite wisdom and calm, Jim's grandfather manages to pay his respects to Mr. Shimerda and avoid the messy religious issue of the suicide while making the Shimerda family happy.
We learn about Jim's grandmother in a similar manner:
She was a spare, tall woman, a little stooped, and she was apt to carry her head thrust forward in an attitude of attention, as if she were looking at something, or listening to something, far away. As I grew older, I came to believe that it was only because she was so often thinking of things that were far away. She was quick-footed and energetic in all her movements. Her voice was high and rather shrill, and she often spoke with an anxious inflection, for she was exceedingly desirous that everything should go with due order and decorum. Her laugh, too, was high, and perhaps a little strident, but there was a lively intelligence in it. She was then fifty-five years old, a strong woman, of unusual endurance. (1.2.5)
The grandmother's character reinforces the gender distinction in My Ántonia – the women are the energetic, lively, strong ones, characters of action and movement who make things happen. While she plays a relatively small role in the novel, she is the one responsible for getting Ántonia out of the fields and into the house where she can learn some manners.
Jim's grandparents, like many other characters in My Ántonia, have a historical basis. Cather based these characters on her father's parents, William and Caroline (source: My Ántonia explanatory notes, Oxford World Classics Edition, 2006). They moved to Nebraska in 1877 and brought with them a bunch of orphaned children. We can see where these fictional characters get their nurturing character traits from.