I found this on the Richard III site and it pretty much deals with the letter and the discussion of any marriage between Richard III and his niece stated as "unnatural copulation."
If this letter really existed (91), and if Buck has cited it fairly, it would be in vain to contend against such testimony, and Elizabeth's fame would be irredeemably affected, not on the ground of her relationship to Richard, but from his being the author of the misfortunes and disgrace of her family, if not the murders of her brothers; and because she had pledged herself but a few months before to marry the Earl of Richmond. The character of Buck as a faithless writer is well known; and even if his notorious inaccuracies and prejudices do not justify the suspicion that the letter itself was never written, it is not too much to suggest that the interpretation which he has given to it is at variance with the truth. As Buck has inserted copies of several documents of much less interest, it may be asked, why did he not give this most important letter at length? Nor is it less remarkable, that even if he were the first person who brought it to light, no other individual should have had sufficient curiosity to copy it. Buck's work appeared in the days of Dugdale, of Anthony Wood, and of several other eminent antiquaries, who have left imperishable monuments of their zeal in collecting historical materials, yet not a single transcript, much less the original of this extraordinary communication, is known to be extant. No other writer than Buck ever saw it, so that its existence rests upon his authority alone, and every one must form his own judgement as to the degree of confidence to which he is entitled (92). The Chroniclers, who impute to Richard the design of marrying his niece, agree in stating that she resolutely opposed his wishes. Grafton's words are, "But because all men, and the maiden herself most of all, detested and abhorred this unlawful and in manner unnatural copulation, he determined to prolong and defer the matter till he were in more quietness;" and this is the only explanation he gives, why, when Queen Anne died in March, 1485, Richard did not execute his design. The Croyland Chronicler, however, offers this additional reason, that twelve doctors in theology gave it as their opinion that the Pope could not legalize it by any dispensation. If this be true, it is not very evident from what source the Pontiff derives the power of authoriizing such an alliance at the present day, even if instances cannot be adduced of the practice at the period in question.
For the reasons which have been stated, it may be presumed tha Richard never contemplated a marriage with Elizabeth; that the letter noticed by Buck is grossly misquoted, even if any letter to that purport was ever written by her; and that the whole tale was invented with the view of blackening Richard's character, to gratify the monarch in whose reign all the contemporary writers who relate it flourished, an opinion which is supported by the fact that not one of them insinuates that Elizabeth consented to the alliance, but agree in stating her utter repugnance to the project