The recent suggestion by a well-known academic that Richard committed incest when he married Anne drew to my mind at least three examples at the highest levels of society where people had done something similar without any criticism.
I was confident that there were many other examples at gentry level. However, as I no longer spend my days with my nose in the pedigrees of Lancashire and Cheshire gentry families, I was not able to give chapter and verse.
However, quite by chance I have found just such an example, from Yorkshire.
“Alice de Scriven was succeeded [as Prioress of Kirklees] by Elizabeth de Staynton, daughter of John de Staynton of Woolley and his wife Joan de Wollay, by whom he had four daughters, Isabel, Elizabeth, Joan and Alice. On John de Staynton’s death his widow Joan married Hugh de Toothill near Rastrick, who, with the consent of his wife, caused Isabel and Joan to marry his two sons, and placed Elizabeth and Alice in Kirklees Priory to be brought up as nuns, so that his two sons might enjoy the whole of the Staynton estate at Woolley, which under their father’s will had been left to his daughters in equal shares.’
The period of this transaction is the 14th Century, and the source is The True History of Robin Hood by J.W. Walker, 1973 reprint, page 116. I know nothing of the author, but have no reason to think him a Ricardian, or to suppose that he invented these marriages in order to mount a future defence of Richard III against an accusation which at that time had not been made.