Article on Joanna of Portugal. She would've been Richard III's 2nd wife had he won at Bosworth. She really wanted to become a nun, but as a princess pawn on the marriage mart, her brother King John refused to allow her to take her final vows even though she had been in a convent for years. The dream to which the article briefly refers has also been termed a vision - Joanna asked John to spend the night in prayer considering the betrothal, and she supposedly saw Richard falling from a horse in battle. The next morning she told her brother of her visions and said that she would agree to become Richard's wife, if he still lived. As Richard was just 32 and in good health, John thought the alliance with England was a done deal. Then the messenger from England came with the news of Richard's defeat and death. John was in awe at Joanna's vision coming true, and permitted her to take her final vows and become a nun.
Since links tend to go wonky in Aimoo, credited to above site and pasted here:
Joan was the daughter of Isabella of Coimbra and Afonso V of Portugal. She was born on 6 February 1452 and she was for short time the heiress presumptive between the death of her older brother John and the birth of her younger brother, the future John II. She was even given the title Princess of Portugal at this time despite the fact that she was only the heiress presumptive, usually she would have been titled as Infanta. Ever after the birth of her younger brother she was referred to as Princess. In 1471 Joan acted as regent in her father’s absence.
Joan was greatly drawn to the religious life but due to her close proximity to the throne her father forbade her from becoming a nun. She was finally able to join the convent after her brother had a child in 1475, but offers of marriage continued to come. A serious candidate was the recently widowed Richard III of England. In a double marriage alliance Joan would marry Richard and Richard’s niece Elizabeth of York would marry the future Manuel I of Portugal. Joan was a descendant of John of Gaunt, through his daughter Philippa of Lancaster. Richard would die in battle a short while later and neither marriage took place. Joan supposedly had a dream that Richard would die in battle. Joan would remain unmarried for the rest of her life.
Joan died on 12 May 1490 at the age of 38 in the convent at Aveiro. Her brother died without surviving issue five years later and had she outlived him she would have become Queen Regnant. Instead her brother was succeeded by their first cousin Manuel. Though Joan was beatified in 1693, she was never canonized so she cannot be considered a saint. However, she is commonly referred to as Princess Saint Joan. She is buried in the convent in Aveiro. Her tomb survives to this day.
As a Tudor aside, Manuel of Portugal would go on to marry 2 of Catherine of Aragon's sisters - Isabella, who died in childbirth with a son who lived only 2 years, and Maria, with whom he had 6 children.