Richard I's nephew Arthur disappeared in his uncle King John's prison, and Arthur's sister was held for 39 years until she died

Arthur, duke of Brittany, was the son of Geoffrey, an elder brother of John. Geoffrey died before Arthur was born in 1187. In 1190 Richard I named Arthur his heir presumptive. When Richard died in 1199 he named John instead. Philip II of France backed Arthur, and Arthur became Philip's vassal. In 1202 Arthur besieged his grandmother, Eleanor of Aquitaine, at Mirebeau. John surprised the besiegers and captured him. Arthur was held at Falaise and then at Rouen. He disappeared in April 1203. It is generally believed John killed him. The Margam annals say John killed him and threw his body into the Seine. His sister Eleanor, called the Fair Maid of Brittany, had her own claim to the English throne and to Brittany. She was in John's hands from about 1202, and in December 1203 he took her to England. She was held at Corfe, Bristol, Gloucester and Marlborough castles. She was never tried or sentenced. Henry III kept her confined after John died in 1216. She died a nun on 10 August 1241, still in custody, and was buried at Amesbury Abbey.