What role does Lucy Steele play in the development of the plot? How does Austen use Lucy Steele to deepen her characterization of both Elinor and Edward?

Exceptionally pretty, Lucy ensnares Edward's affection while he is her uncle's pupil. She herself has little education and no money and is glad of this opportunity to come up in the world. When Sir John invites Lucy and her sister Anne to Barton Park, the sisters are shrewd enough to bring presents for the children and to flatter their mother into thinking they dote on them. Thus they soon win her favor.