Edmund was the only person who was nice to Fanny when she came to Mansfield Park. He helped to teach her. When she became a young teenager, she fell in love with him. Her love was not obvious because she was shy and the Bertrams were expected to treat Fanny like a sister. When Edmund became an adult and began to love Mary Crawford, he was deceived by her true nature. Yes, she was very pretty and rich but she wasn't as decent as Edmund or Fanny. Through most of the novel, Fanny was in love with Edmund but it was not until Edmund realized the true nature of Mary that she lost his respect.

Loving, guiding, protecting her (Fanny), as he (Edmund) had been doing ever since her being ten years old, her mind in so great a degree formed by his care, and her comfort depending on his kindness, an object to him of such close and peculiar interest, dearer by all his own importance with her than any one else at Mansfield, what was there now to add, but that he should learn to prefer soft light eyes to sparkling dark ones. And being always with her, and always talking confidentially, and his feelings exactly in that favourable state which a recent disappointment gives, those soft light eyes could not be very long in obtaining the pre–eminence. ..... She (Fanny) was of course only too good for him (Edmund); but as nobody minds having what is too good for them, he was very steadily earnest in the pursuit of the blessing, and it was not possible that encouragement from her should be long wanting. Timid, anxious, doubting as she was, it was still impossible that such tenderness as hers should not, at times, hold out the strongest hope of success, though it remained for a later period to tell him the whole delightful and astonishing truth (that she was in love with him for years).