I don't think that kind of distinction mines any deep meaning from superstition and cruelty.
Doylist (reality/metatextual): Sadists and cultures beat on scapegoats instead of address core injustice, which makes them easy bad-guys to cheer against. We miss our own societal cruelty blind spots. Systemic abuse is an adornment to the sisters getting abused. Starvation made Claudette a worse pawn, but it fits the theme.
Watsonian (in-universe): A bunch of sadists scapegoating a little girl because thinking hard and power comfy. No one intervened because Claudette was a dirty secret who was expected to be murdered, and if Marietta died it's not like her identity had to be reused.
You were right about the fear-based prejudice but the central issue is logic (ethics and politics this time). People, especially the maids, felt benefits from abusing a child. Which made Claudette impossible to swap with her sister (which was probably unnecessary anyhow). There's not enough layers for it to matter that "we live in a society"; no maid sob-story, no historical deep dives, just systemic excuses for personal excess.
Not to say it doesn't work. The chapter made me crave more vengeance and revolution than I expect from the story. Claudette made some severe mistakes, she's probably going to lose her mentor too for max angst, and she's in a good position to be an antihero villainess. Down with the Empire. And the Snewe Kingdom. She should get her summoned dragon forging guillotines posthaste... assuming the guardian beast is loyal to her and she's not stewing for years on end again.
That's pretty much what I was getting at.
Maybe my initial mistake was assuming the original person who posed the question I'd answered, wasn't asking rhetorically. But I just gave my read on
why, in-universe, Claudette was treated the way she was.
And we even see the second maid admonish the first when Claudette is branded with the iron. So it's clear that, despite her abuse and mistreatment, it was still presumed that Claudette was to not be excessively harmed - at least until it was revealed that we were at the point where Marietta was being introduced to her eventual betrothed, meaning Claudette had outlived her potential "usefulness".
So that would indicate that Claudette was seen as an eyesore that brought down the "standing" of those who were (in their view) forced to care for her, and that resentment became actual violence and abuse borne of those sentiments. But those resentments have their roots in the precedent of the 'bad omen' that is twins within the royal bloodline, which - "systemic excuses" or not - is what we're shown as their justification.
And that's the whole of my initial explanation. I do not condone what they did to Claudette, and I certainly don't think it makes logical sense to leave her malnourished and constantly injured/disfigured and (likely) ignorant to the things a princess should actually know. I merely responded to "why would they behave like this in-setting", and left everything else that didn't pertain to that, aside.
-
To your last bit - I wonder if Claudette is going to be working, if not at odds with Marietta, then simply independently of her, and will rebuff Marietta's initial efforts at connection and reconciliation between them. Marietta
is rather passive and gentle, and so while I suspect she will be very upset over what's happened to her twin sister, I also imagine that she would not want to resort to immediately burning the Snewe kingdom to the ground - her upbringing wasn't perfect, but she was happy and had loving parents in her memories, and would likely suffer some dissonance that those people could have treated their other daughter as they did.
So that poses a likely clash between them - Marietta wants to help Claudette heal, but Claudette's interested in revenge. I kinda think that the actual antagonists will be kingdoms and systems like the nobility, not necessarily individuals like the their royal parents.
But I am intrigued in how Marietta will navigate trying to fix things as she sees them broken, while Claudette (likely) tries to tear everything down, and chaos continues to unfold once the Swap is revealed and the Snewe kingdom, and their alliance borne of the marriage of their princess, begins to unravel underneath it all.
