In chapter 15, Sara (and the reader) enters an emotional roller-coaster of an evening. Fresh from the episode of the fourpence, the hot buns, and the beggar girl, she arrives back at the school only to become the target of secondhand rage. The cook has been berated by Miss Minchin--as we later learn, deservedly so for feeding Miss Minchin’s special dinner to her gentleman friend and then blaming the disappearance on Becky. Shit, as they say, rolls downhill.
We regularly see the question of whether one returns evil for evil or good for evil. When Cook gets crap, she’s the sort to turn around and dump it on someone else: Becky, Sara, anyone who can’t fight back. But that doesn’t mean that every time Sara returns good (or at least good behavior) for evil, that it’s out of pure virtue. Sara can be somewhat agressive in her obedience and passivity. Her internal monologue about how she only puts up with crap "because she’s a princess and that’s what princesses do and wouldn't they all be terrified if they realized she was really a princess and could have them all beheaded" can’t exactly be characterized as meek obedience.
But tonight we see a different view. Having been scolded and refused dinner out of pique, her impulse on finding that Ermengarde has visited her in the attic is to do her utmost to protect her friend from any knowledge of the bad parts of her life. And that, in the face of Ermengarde’s offhand disinterest in things Sara is starving for--both the intellectual nourishment of the gift of books that Ermengarde considers a burden, and the physical nourishment of the “care package” of food that Ermengarde had completely forgotten about.
It isn’t until the two girls eavesdrop on Becky being scolded and slapped up the stairs (due to Cook’s accusation), and Sara explodes in indignation about how hungry Becky always is and yet she’d never steal food, that it finally occurs to Ermengarde that Sara’s thinness is due to actual starvation. Unlike Sara, whose first tangible gifts to Becky are food, Ermengarde hasn’t the imagination to realize Sara might be actually hungry--nor even the reflexive impulse to share her box of food with her best friend, hungry or no.
But Sara, on being offered a share, immediately reqeusts that the invitation be extended to include Becky and then gives back in equal measure by turning what could have been a furtive snack into a dramatic historical pageant. Both imagination and a few props made from rubbish turn the attic into a castle banquet hall. [*] It occurs to me that one of the reasons for framing the feast as "banquet for a visiting princess" is to remove it from the realm of pure charity. Pretending a castle banquet hall moves the act of nourishment into the realm of make-believe. It allows Sara to give back in equal measure for what she receives. That act simultaneously allows Sara to reject the role of starving beggar (a role we've seen her reject previously in turning the Carmichael boy's sixpence into an amulet rather than spending it on food) and to remove some of Ermengarde's awkwardness in her belated understanding of Sara's situation.
The roller-coaster, having dipped low, now rides high. But the deepest plunge is just about to come...
[*] Periodically, I take note of what might seem trivial implausibilities, such as the question of exactly where Captain Crewe’s fortune came from. One such implausibility is the brief fire that Sara lights in the attic grate to provide the illusion of heat and light. If--as we are told--there has been no fire in that grate for a very long time, the likelihood is low that lighting some trash in it would result in a cheery blaze rather than billowing smoke. Even assuming that the text simply omits the step where Sara opens the damper on the flue (because if there isn’t one, then the attic would be utterly freezing in cold weather), unused chimneys of that era were notorious for collecting birds’ nests and other blockages. You just don’t light a big heap of paper and rubbish in a long disused fireplace and expect success.