This was a very interesting chapter!
Though, I do have to shake my head at Karina's mother having the same delusion that I've seen in a lot of other stories with a sickly younger child (Looking at you "My Fiancee Is In Love with My Sister").
From the mother's perspective, Karina was a "loving" child because she never complained when things rightly given to her were instead given to Aurora. It isn't hard to see how, after years of parental pressure, Karina simply stopped trying to voice her concerns. I'm sure in the spaces between there would have been times where Karina told her mother and/or father, "No," but, they would have been slapped down with a simply, "Think of your sister. You are destined to be royalty, and Aurora is so very sick".
I have my fingers crossed that Karina will eventually confront her parents to throw her extreme displeasure back in their faces at having spoiled Aurora and left Karina in the dust (something like, ironically enough, "Karina's Last Days" [which, according to the raws, is up to chapter 86 {here's hoping Tapas starts translating it again!}]).
Seeing her mom in mourning clothes still, I think it comes down to the guilt catching up to her, she likely FINALLY asked herself WHAT was Karina favorite things, ETC, and realizing, she knew NOTHING, she knew so much about Aurora, but best she knew of Karina was "she was a good girl who loved her little sister and would do anything for her" (AKA the DELUSION she convinced herself of, as you said about her and her husband likely SLAPPING Karina's ability to say No down, it's TYPICAL for Neglectful/abusive parents to try to "change history" where they WEREN'T crappy parents!) I know her words ANNOY people. But I think it's best to say she wants to discover the TRUTH of what was happening behind the Royal Family's "Closed doors" as her clothes show, she's in GRIEF and wants to know the full truth, likely she wasn't even ALLOWED to mourn her daughter properly when the King came and likely told her and her husband to "remove all evidence of her existance" while her husband likely COMPLED, it caused division between them both as he likely was on the "so what, Karina fucked up ROYALLY, Aurora is going to be queen, why fuss, I'll just make a SECRET GRAVE for her we can mourn at and say it was some distant cousin we were once close to, it's Karina's own fault this is happening to her, why should we all go down with her! Good thing Aurora was able to FIX THINGS" with her getting MORE upset because as her mother, she KNOWS something was wrong, likely was "pushed" by her husband to treat Karina this way and because Karina "did as she was told" she saw nothing wrong till all this shit happened. It's as THOUGH the royal family said one thing, let her and her husband know NOTHING till everything happened, and as she went around, like the Scribe all she got was "her daughter was EVIL" but like the Scribe, you saw stuff "slip through" that makes you question WAS she really THIS EVIL as she said, "why would they let her REMAIN Crown Princess?!" Wouldn't it have been better to EXILE her back to her family home or to NUNNARY as punishment.....unless they FEARED information she KNEW that was SENSITIVE enough for them to ENCOURAGE the idiot prince to EXILE her somewhere where "it wouldn't be STRANGE for her to SUFFER an ACCIDENT!"
I feel it's a MIX of both with the Conspiracy on her "EVIL" AKA much of her EVILS may have been BACKED, the son likely COMPLAINED he didn't get AURORA early and had to "hold his tongue" for the sake of them USING her, likely as a SCAPEGOAT for other CRIMES the Royal Family were doing. It's likely EVEN IF Aurora never "recovered and remained sickly forever" Karina STILL would have been made a SCAPEGOAT to cover up for them. Why was she allowed to treat foreign dignataries like crap? You'd think they'd be upset over that, unless she was ORDERED to do so, so they can BLAME HER for it while BENEFITING in a different way (seeing how INCOMPETENT and short tempered the Prince is, My Guess was to SELL HIM as "better then Karina, they'd blame it ALL on Karina, including HIS FUCK UPS and Aurora gives off the vibe of "a good queen" who will make everyone FORGET how incompetent the Prince is! Factors likely will be put in place as to make him a FIGUREHEAD! Hell who's to not say the King wouldn't "still rule even in RETIREMENT" with his son just told to "Follow the scribe and be happy to be KING!" Karina would in that scenario STILL be QUEEN, but be seen as "the reason bad stuff happened!" Basically make her INFAMY CLOUD UP the loser Prince's own Infamy!
I doubt Aurora denied the engagement and was likely WANTING to marry asshat and was shocked she was made the Fiancee, but her parents convinced her "otherwise" or likely CONVINCED HER seeing how she is that this was ALL some plan to hold the spot for her, hence why she never saw herself as "stealing" because everything was "Set up" and Karina might have "gone too far" without EVER KNOWING that everything she had or was given at Karina expense were that and they DIDN'T have a good relationship (Sort of reminds me of two SISTER characters from a Manga I'm reading, "Shinu Unmei ni Aru Akuyaku Reijou no Ani ni Tensei shitanode, Imouto wo Sodatete Mirai wo Kaetai to Omoimasu" or well the NOVEL version at least! Where turns out the sister with the MC of the story was ABUSED by her parents and servants, but her sister who never WITNESS the bad behavior ONLY remembers their parents and those same servants "fondly" and can't FATHOM other then "blaming the MC" why she wouldn't want to "live with her" after reuniting! Cause she can't get that she spend ALL that time being abused and always COMPARED, her sister is a case of Unwittingly Insensitive. The ultimate Irony is the reason of their HOME being destroyed in the first place was because the son of a KNIGHT Commander who was suppose to be the OG story "Knight Fishy" for the Heroine in the Otome, had fallen in love with her, but his father was RACISTS to beastman which she was, though that was a secret, and HE was the one to arrange it all! You feel bad for him cause a simply "wish to SAVE her from her bad parents" as a kid lead to her nearly suffering a fate worse then DEATH if not for MC interference! Fucked up cause he was a KID asking to get ENGAGED to her! Yet his father decided instead of telling him NO or letting it go, decided to destroy her whole house.....the same HOUSE that sans her SISTER treated her like crap)
It will be funny if the Scribe meets her, other then the fact SHE WAS ALIVE all this time. At least hearing HER SIDE. Of course this will bring trouble since the Royal family were CONTEND with her "Dead and forgotten" so to find out she DIDN'T die under that snow and was still around. That means ALL the secrets she knows are "Still out there!" The Campaign to "Erase her" likely so no one thinks any further then "she was so bad she had to be ERASED for the GREATER GOOD! I wouldn't be surprised if her REVENGE is simply doing what they FEARED she would do, that brings THEM down
