By the time Fanny reaches adulthood, her love for Edmund is a foregone conclusion. At the time, however, Edmund’s attention is distracted by Mary Crawford, who appears to be everything that Fanny is not. While Fanny is shy and reserved, Mary is vivacious and outgoing. As is always the case with unrequited love, the reader can scarcely help feeling desperately sorry for Fanny as she watches Edmund and Mary grow closer.

It’s not, however, as though Fanny is invisible to Edmund: she is his friend and his confidant at this stage of the story. The fact that Edmund’s feelings for Fanny did not initially extend past friendship can be seen in the fact that when Henry proposes to Fanny, Edmund urges her to accept him.

As the old saying goes, absence makes the heart grow fonder, perhaps demonstrated by the time in which Fanny goes to stay with her own family for a period. During this time, Edmund’s infatuation with Mary ends, and he finally sees Fanny as the woman he should marry. Fanny has clearly been subtle in her affections, because Edmund takes the time to strategize about how he could convince Fanny to love him, without realizing that she had already loved him for so long.

Fanny’s relationship with Edmund, therefore, develops from friendship and unrequited love to mutual affection and marriage.

Fanny and Edmund finally marry—but, through much of the novel, it is not apparent that their relationship will turn out this way. Edmund is six years older and thinks he loves another woman. Fanny is a child when the novel starts and has to grow up in many ways. It seems that she might marry Henry; Edmund, for a while, promotes that match.

Edmund, when the action opens, is contemplating becoming a clergyman. This was then a typical choice for an upper-class family’s second son, which he is. As a serious, honest, and kind person, his relationship to his younger cousin resembles that of a brother. When he later tries to convince Fanny that marrying Henry would be a good idea, he seems sincere. At that point, Fanny’s reasons for turning down Henry are not clear to Edmund, even though the reader understands that Fanny loves Edmund. Mary Crawford, the object of Edmund’s affection, turns out to have numerous character flaws, to which Edmund finally opens his eyes.

Fanny, who is only ten when the story begins, was taken in by the Bertram family as a child; she suffers the burden of being a poor relation. As her father had been a sailor and she has no fortune of her own, her prospects for a successful match are limited. While in virtue she outshines them, in practice she is overshadowed by the Bertram daughters, who are favored by the father despite their shallowness. Sir Thomas pushes her to marry Henry, but she refuses. This stubborn behavior—unlike her usual docility—shocks the family.

During a period when Fanny is staying with her own family, Edmund confides in her while he is deciding to break with Mary. Once he is over that infatuation, he starts to see Fanny as an ideal wife and figures out how to persuade her to love him in return—not realizing that she already does. Austen does not describe the proposal and acceptance, rather saying Edmund felt “delightful happiness” and that she would not presume to describe Fanny’s happiness. Sir Thomas accepts the match, and they are married.

When Fanny arrives at Mansfield Park as a frightened 10-year-old, Edmund is the one person in the family who befriends her. By 18, she has fallen in love with Edmund. However, just at this time, a rival, Mary Crawford, appears on the scene. While Fanny has led a timid, quiet life, Mary is beautiful, witty and worldly and quickly catches Edmund's eye. Fanny can do nothing but wait silently for the blow to fall and Edmund to propose to Mary. In chapter 11, Fanny endures the "mortification of seeing him advance ... by gentle degrees" towards Mary, while she, Fanny, "sighed alone at the window." 

Fanny's only hope lies in the fact that Mary does not want to marry a clergyman, which is Edmund's chosen profession. Because she is such a close friend, Edmund often confides in Fanny about his concerns over Mary's morals and upbringing.

The story takes an unexpected turn when Mary's brother Henry falls in love with Fanny. Fanny loathes Henry as a person of loose morals, a man described by Austen as "ruined by early independence and bad domestic example." When Henry, who is very wealthy, proposes, Edmund urges Fanny to accept. Fanny refuses and is exiled to her family in Portsmouth to think it over. She has begun to weaken when she learns Henry has eloped with her married cousin Maria. Mary takes her brother's side and at that point Edmund breaks with Mary.

Edmund and Fanny both arrive back at Mansfield and at "exactly at the time when it was quite natural that it should be so, and not a week earlier, Edmund did cease to care about Miss Crawford, and became as anxious to marry Fanny as Fanny herself could desire."

Although Fanny and Edmund are first cousins, they marry: for cousins to wed was not considered odd at the time.