aurora cant have planned for this outcome, her elder sister was sentenced to death by assassination by the royal family and almost killed as a defenseless victim in the wilderness.
karina meeting reshutoka was built up as a coincidence.
furthermore chapter 6 states that karina has been officially reported as dead after the avalanche.
making a plot twist to present the little sister as selfless and protective will end up being very hypocritical.
oblivious might be more believable.
It isn't necessarily a dichotomy of "mastermind savior" and "evil villainous sister", though - or even "savior" and "oblivious".
She was sickly since childhood; Karina was even the stand-in Consort in her place, because of it. And, as
@Kinkaiser stated above, Aurora stopped the prince from sending soldiers, instead sending gifts and servants to "make the castle more habitable", at least in her own mind. So we've gotten no outright antagonistic behavior or stated intentions from Aurora this entire time, but while she seems rather passive and blank, I think it can be argued that there are hints that she's aware of more than she's letting on.
If I had to guess - I think that Aurora is not the villain that Karina thinks her to be, and has simply tried to find some way to get Karina out of the engagement that she likely believes that Karina never wanted.
Karina was obviously suffering in some fashion during that time, and Aurora, who was supposed to be the one to marry into the Crown in the first place, could have wanted to spare her sister more pain, and did what she was able from a limited place of influence. She couldn't have planned the meeting with Reshtoka, or the subsequent destruction of that castle in the avalanche, *but that doesn't mean Aurora is wholly separate from Karina's fate or has no interest, vested or otherwise, in the wellbeing of her sister.
The Crown scrubbing Karina from the history books isn't necessarily something she could influence either, given she's even now not officially Royalty, and wouldn't have the sway to prevent something like that, especially if she had to hide mourning her sister's presumed death from her fiance.