For at least the last three generations of the royal family, they are seemingly cursed—or chose—to have only one heir. Duncan, Cassandra, Maddie, all an only child. But evidence could suggest that Duncan has a sister.
Oswald and Deborah are getting old in 'The Early Years,' so much so that it's not a leap for people to believe Oswald is losing his touch naturally, rather than being poisoned. Deborah's life is mostly a mystery, but she is described to have gray hair and a wrinkled face. In 'The Battle for Skandia' Cassandra makes a comment on Deborah's appearance born from seeing her grandmother alive, though Deborah is no longer alive and didn't seem to have passed away recently. Duncan was born when his parents were getting to the upper end of being able to conceive. This seems irresponsible, as Duncan is believed too young to take on the burden of the crown, giving Moragarth more sway over the barons.
Duncan is very young when he gets married and begins the process of producing heirs. Whether he wanted more children becomes a moot point, though, when his wife dies in childbirth. Cassandra is older, low 30's, when she gives birth to Maddie, but still young enough to have more children if she wants. Unless Deborah and Oswald got married in their 40's, they had time to have multiple children, or produce one heir when they were younger and more energetic, with Duncan as a happy accident—Flanagan's universe is still net positive enough to remove the hardships of conceiving or infant death.
Duncan could have had a brother, one who encountered bad luck on the battlefield and passed away. Princes are expected to do military tours as youths, a common complaint of Cassandra's when she's stuck embroidering and planning dinners in the castle.
A sister, on the other hand, could still be alive. Oswald never lived long enough to know Duncan only had a daughter, yet changed the rule of inheritance to include a Queen. This is a long-standing disgruntlement of 'The Red Fox Clan,' who twisted facts to believe it was done for Cassandra's sake. An Araluen king has the right to declare a new heir, even as far as going outside of the family, as Morgarath hoped to trick barons into believing with forgery.
Women are treated terribly in the series: Sir Montague, robbers, and evil soldiers all think of women as lesser and destined for servitude. Duncan's sister may have been married to a royal from another country to secure a treaty after Duncan was born, but before the laws of inheritance could be pushed past dissenters, and Oswald made the change for future generations of royalty in case Duncan didn't produce a male heir.
Maddie has a rogue cousin in Dimon, the boy angry towards the royal family through his mother, bolstering the Red Fox Clan. This is through Deborah, who didn't enjoy castle life and often separated herself from it, hence Morgarath being able to step in and kidnap Oswald before she could stop it. However, if that strand is bitter towards the Crown, those weren't the cousins Cassandra played with as a kid, mentioned in 'The Burning Bridge.'
Also in that book is the reason she is heading to Celtica: to visit her friend, the princess, in a guaranteed safe location for the duration of the war. Celtica is a stable border and they have a mutual defense treaty, unlike the contentious nature of the border with Picta. Cassandra's connection to Celtica shows she visits, the Crown not just sending diplomats to communicate, and if Duncan's sister is married to King Swyddned, that explains how Cassandra has cousins who cannot inherit the Araluen throne and Celtica would make sure Cassandra remains in perfect health.
The cousins are a pretty convincing piece of the puzzle for Duncan to have a sibling. An alternate explanation to Oswald ensuring female inherence, though, is that Deborah could have been the royal bloodline, who then had to give up the throne to her husband. That would make Deborah's extended family upset, and over a few generations garble the bitterness into Dimon wanting the throne as the male heir. Surely if Oswald were the bloodline, it would be his side of the family to be in line for the throne?
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