Here's a weird minor factoids: Maid Marian is the King's "niece" in the movie. But she doesn't seem to be related to him or John directly. So King Richard would have to have married Marian's mother or father's sister for them to be related at all.
The actual Richard the Lionhearted married Berengaria of Navarre, who is famously known as the only queen to have never set foot in England, having married the King during his crusades. She did indeed have several sisters, although there's barely any information on any of them. Blanche, the wife to the Count of Champagne (who died in his early 20s), had one girl, whose name supposedly was Marie. She also had a son before the father died, who only became Count at 21 after a long ass war over the position. Because of that dispute, one of the conditions in the treaty with Philip II after the war was that Marie had to be sent away to the royal court in Paris. So if there's any historical precedent for who Maid Marian was, it was probably Marie. Although it seems like absolutely none of the family ever entered England at all. Other than maybe Richard's wife, the queen, briefly after his death.
