What is the development of Fanny's relationship with Edmund in Mansfield Park?

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...

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.