A relationship that stands out is that between Edmund Bertram and Mary Crawford. The two meet after Mr. Norris dies, and the Grants move into the parsonage. Mary is a younger half-sister of Mrs. Grant, who stays with her.
Edmund quickly falls under the spell of the lovely, witty, talented, and ever-entertaining Miss Crawford. Fanny, through whose eyes the story is told, watches the relationship unfold with dismay. She is primed to be prejudiced against Mary because Fanny loves Edmund and sees the other woman as a rival. Nevertheless, Mary's worldly views and tolerance of vice are stumbling blocks to the relationship. Much of the novel's plot turns on whether Edmund marries the dynamic Mary or the virtuous Fanny.
The relationship is meant to be entertaining but, more than that, to serve a didactic or cautionary purpose. Mary has been brought up badly in the household of her admiral uncle and exposed to so much immorality at an early age that it has damaged her in Austen's eyes. Because her values were warped early in life, she throws away happiness with both hands when she finds it in Edmund. She values money and a worldly position too much: she loves Edmund but also wants him to pursue a career other than country clergyman, one with more money and prestige so that they can shine as a couple in London society. She also wants to brush over Maria and Henry's adultery: she is not concerned with it, only with hiding it. Her inability to value what is most important in life leads her to lose a chance for a deeper happiness with Edmund that transcends the shallow world she has always known.
Mary, it should be noted, is a controversial character. Another reading would be that this judgment of Mary as warped is Fanny's condemnation of her rival and that Mary is the better of the two, who escaped marrying a prig in Edmund. In either case, differences in moral values between the two characters are explored for didactic purposes.