In Mansfield Park, the relationship between Mary and Edmund serves as a catalyst for Edmund’s character development.
Edmund is the younger son who will never inherit his father’s property. He meets Fanny when she comes to live with the Bertrams as a young child. They grow up together, and Edmund is her sole confidante and true friend. As they grow into young adulthood, the difference in the respective status becomes clear. Edmund is destined for the clergy or another respectable role befitting a younger son, while Fanny is expected to remain at Mansfield Park as the helper to her aunts.
However, Fanny’s status seems to improve when Mary’s brother Henry Crawford falls in love with her. Her Uncle Bertram is in favor of the match and pushes her to accept Henry’s proposal. When she refuses, she is forced to leave Mansfield Park and return to her parent’s home.
Back at Mansfield Park, Edmund continues his relationship with Mary, although he misses Fanny terribly. When his older brother falls ill, he is sent to retrieve Fanny so that she can help nurse the brother and assist her Aunt Bertram.
Fanny has realized that she no longer belongs in her parent’s world. She has grown up accustomed to luxuries and, more importantly, to books and education. By comparison, they are uncultured and live in relative poverty. She briefly accepts Henry, but then changes her mind shortly thereafter. Her rejection pushes Henry to run off with Maria Bertram, Edmund’s recently married sister.
Like Lydia in Pride and Prejudice, Maria’s rash action casts her outside of polite society. Mary counsels the Bertrams to accept Maria back into their home, which will cause others to accept her over time and she will regain some measure of respectability. Mary’s advice is self-serving. She wishes to marry Edmund and would rather marry a man whose sister has not fallen into disgrace. Edmund recognizes this and confesses to Fanny that he loves her. When the book opens, Edmund is a thoughtful character who encourages Fanny to learn and grow, but he is dazzled by Mary’s charm. As the book progresses, he grows and recognizes Mary’s shortcomings and Fanny’s virtues.