Based on descriptions in Hattie Big Sky, by Kirby Larson, we have to make some assumptions about Aunt Ivy's appearance. However, we do know that Aunt Ivy wants to be seen as a certain kind of person, and takes pains to ensure that others view her that way.
For example, in Hattie's first letter to Charlie, she writes: "You should see Aunt Ivy. She's made herself a cunning white envelope of a hat with a bright red cross stitched on the edge. She wears it to all the Red Cross meetings. Guess she wants to make sure everybody knows she's a paid-up member."
Clearly, Aunt Ivy wants others to know how deeply invested she is in the war effort. She wears an outward symbol—the Red Cross hat—to demonstrate this. Other than that, we have to infer other details about Aunt Ivy. She is described as "holding court" in her brown leather chair, so we imagine a strict woman who takes charge of her surroundings. Later, she is described as smoothing her skirt, so we can assume she likes to be dressed crisply and wrinkle-free. All in all, the author gives us the impression that Aunt Ivy is a stern, humorless, and careful person, who takes her appearance seriously and has no sense of fun.