So this is that same grandmother, huh.

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So this is that same grandmother, huh.
So... In TEY, most of the kingdom believes that king Oswald is being protected by Morgarath and that prince Duncan is the big baddie raiding the villages on the north, right?
His mother and lady Rosalind probably believe that as well
Maybe they doubt that it really is Duncan, especially if they know him well (Deborah is his mum and if he was already seeing Rosalind...). They doubt that it is really Duncan but they have no way to contact him. They don't know it's a substitute and the real prince is a prisoner at castle Wildriver. And with every new message about fake Duncan's rampage, their faith in him slowly disappears... They don't know. They stop believing in him. And Duncan doesn't know that.
The Araluen royal family seems very small - riskily so. Royalty and nobility usually try to have at least several children, so in case the heir dies - the risk of dying young was a lot more common back in the old days, between sickness and accidents - there's still other family members to inherit if needed.
But Duncan has no siblings. Cassandra has no siblings. Madelyn has no siblings.
We don't know anything about Duncan's childhood. Maybe he did have more siblings who died. (I personally headcanon that Oswald and Deborah got married later for whatever reason so Duncan was the only child they had [which is part of the reason why Oswald changed the law to allow girls to inherit, because if he and Deborah only had a daughter and couldn't have any more children than someone needed to rule the kingdom] , but that's just my headcanon.)
But we know about Cassandra, and we know she's definitely an only child. Alright, so we know why Cassandra is an only child. Her mother died in childbirth. Maybe her father was too brokenhearted to ever remarry. Not to mention that what with having to deal with the aftermath of Morgarath's war, build himself a reputation as a good, strong king (with Morgarath not having actually been defeated, and with the army unable to get him out of the mountains, that probably didn't look very good for Duncan's first war as king) and raise a newborn daughter, remarrying was probably not high on his list of priorities at the moment.
But after a few years, even if Duncan himself didn't want to remarry, his court should have been beginning to urge him to. After all, Duncan only had the one heir. If anything happened to Cassandra, there'd be no one to inherit the throne. And indeed, Cassandra came close to dying many times. What if she'd died in Celtica? What if she'd died in Skandia? The future of Araluen's throne would have been in jeopardy. It would've been a lot less dangerous and risky if she'd had one or two siblings, just in case.
Moving on from Cassandra, there's Maddie, who is an only child as well. Again, it's risky for the monarchy to have only one heir. Especially now that she's no longer living safely at the palace but is training as a Ranger. Sure, she's not planning on actually becoming an active Ranger, but Ranger training in and of itself can be risky. Ranger apprentices are expected to go on missions and risk their lives just like full Rangers. Maddie almost died when she was wounded in the hip that time. What if she actually had died? What if she dies from an accident during training? All that would be needed is one bandit or whoever to get in a lucky shot, and that's it.
Royal families usually try to have at least "an heir and a spare." Just in case something happens to the heir, that's what the spare is for. But Araluen hasn't had any "spare" in generations.
The kingdom needs heirs, and generation after generation of only children in the royal family aren't enough of them to secure a stable future for Araluen if there is no direct heir. What if Maddie dies, and Cassandra at this point is likely too old to have children? Who will inherit the throne after Cassandra?
So that's where Cassandra and Maddie get it from.
I mean, Duncan is a skilled swordfighter, but is never shown using any long-range weapons, unlike both his daughter and his granddaughter...