When Phoenix arrives at the doctor's office, the nurse informs the attendant why Phoenix is there:
"Oh, that's just old Aunt Phoenix," she said. "She doesn't come for herself--she has a little grandson. She makes these trips just as regular as clockwork.
The attendant would have assumed that the ancient Phoenix was at the doctor's office for herself. The nurse clarifies why she is there. But this also indicates Phoenix's selflessness when it comes to her grandson. Even in her old age, she makes the trip like "clockwork" to get his medicine. She is clearly dedicated to him and this routine.
When the nurse asks Phoenix if her grandson is okay, Phoenix spaces out and has a temporary loss of memory. When the nurse asks again, Phoenix snaps out of it and says he's okay but in need of more medicine. The nurse says it's an "obstinate case" indicating that the boy will never get better; yet, Phoenix continues to care for him. The nurse gives Phoenix the medicine out of a sense of duty: Phoenix continues to make the journey to get the medicine out of a sense of love.
Phoenix says, "We is the only two left in the world." They are the only two left of their family. This could also mean that she has a special bond with her grandson; to her, they are the only two in the world.
The journey to the doctor's office is enough to show Phoenix's love for her grandson. Her last gesture of love in this story is when she decides to buy him the windmill with the two nickels she acquired on her journey. Notice that she doesn't use them for herself.