Who is Aunt Harriet from The Lion's Lady and what is their importance?

Aunt Harriet is Millicent's sister, and she travels to London to oversee Christina's social season. During her first meeting with Christina, Christina and Lyon are quarreling. Thinking that the visitor is Lyon, Christina begins yelling at the woman. Aunt Harriet says that she is impressed by Christina's spunk for having the nerve to yell at Lyon.