Hm, as this is not only about Britain obviously:
Helene Duchess in Bavaria should become the bride of Emperor Franz Joseph of Austria while her younger sister was meant for the emperor's younger brother, Karl. Instead Franz Joseph took the younger sister Elisabeth. Helene became princess of Thurn & Taxis while Karl married Margarethe of Saxony, a cousin of both himself and Elisabeth of Bavaria as their mothers were sisters.
Maria Elisabeth of Austria, daughter of empress Maria Theresia was engaged to be married to king Louis XV. of France. When she got sick with pox and became disfigured through the disease, the king refused her and asked instead for (or was offered) her sister Maria Antonia's hand in marriage for his grandson, then the dauphin (later king Louis XVI.)
Quite extreme the example of their sisters. First Maria Johanna Gabrielle became engaged to king Ferdinand I. of Naples as a child. As she died before the marriage, the next sister Maria Josepha was betrothed to the king. As she died on the day her wedding should have been celebrated, the next sister Maria Karolina became queen of Naples. It's not exactly that, but it shows how it worked -another Habsburg-example: their brother Joseph had been married to Isabelle of Parma and happily so, so he forced his sister Maria Amalia to marry his wife's brother duke Ferdinand of Parma: a very unhappy marriage.
I'm sure we can find some more examples if we tried hard enough, but these came immediately to mind.
In the Middle Ages/renaissance it was quite common for a child bride to be sent to the court of her future in-laws to be raised there. But if the groom died and the bride was of political interest, she was married by the next in line. Alternatively the bride could be married to a minor member of the family if her appeal changed over the years and her family wouldn't take her back. Margarethe of Austria, daughter of emperor Maximilian comes to mind. She was raised from the age of 3 at the French court to become the queen of king Charles VIII. but he took Anne de Bretagne instead. Margarethe was offered another French marriage but her father did not allow it. Only after two years of negociations was Margarethe allowed to leave France and became the wife of the heir to Ferdinand of Aragon and Isabella of Castile, Juan. Within the year, Margarethe was a widow. Back in Germany, her father convinced her to become the wife of the duke of Savoy. When only aged 24 she was widowed again she decided to stay that way and did not contract another marriage.
Her niece Eleonore of Spain and Austria became the third wife of Manuel I of Portugal (after her aunts, the sisters Isabella and Maria of Spain = sisters of Catherine of Aragon and Juana the Mad had subsequently died) and after becoming Manuel's widow she married the king of France...
So what happened to Catherine of Aragon was quite common...
