The relationship between John Keats and Fanny Brawne was not well published nor was it successful--for whatever reason. There is no doubt that she impacted Keat’s life, and while it is questionable how he affected her life, some research has shown that Fanny Brawne did admit to those close to her (unbeknownst to her husband) that she had been involved to some extent Keats.
The following website has interesting information and links.
Fanny Brawne was a strange one - after John Keats death she wore special mourning clothes for him for several years, but then went on to get married in 1833. I'm not saying that was wrong, just that judging by the see-saw passion of Keats for her - I don't think he would done that if it had been her that died first. maybe not 100% reciprocal?
Keats died at Rome in February,1821.He was buried in the protestant Cemetery,near the tomb of Caius Cestius,the 'one keen pyramid with wedge sublime,' described in Adonais.The epitaph was chosen by Keats himself,in a moment of bitter despair: 'Here lies one whose name was writ in water.' To what extent the epitaph proved true ?
During the last few months of life,Keats was afraid to write letters to Fannyand even the sight of her handwriting was emotionally upsetting to him.He did not read the last note,he recieved from her,instead he asked that it be placed in his coffin along with a lock of her hair.This shows keats' devotion to Fanny Browne was complete.
The tempestuous relationship would both enthrall and torment him for the rest of his short life.
I don't think there is any indication that letters exist from Fanny to John. That doesn't mean that there aren't any that may perhaps be discivered. There is also very little evidence other than the letters from him that explore their relationship.
only 39 letters written by keats to fanny are acknowledged and some published also.But none of fanny's letters survived ,which she wrote to keats.so this subject has remained a point of discussion and speculation.