The parents weren't subtle about having a favorite child. If I was karina, I wouldn't want anything but revenge for being viewed and treated as a tool for political and financial gain. Karina may not understand why her little sister did what she did, but it was the only card she had to free karina from being used by society. Whatever karina had, the sister wanted and karina ended up having to give it to her. Karina's whole life became, live for the family's benefit, no fun and happiness for you. P.S. "we like your sister more and she gets to have fun and a childhood, deal with it loser..."
If karina was truly causing problems in the capital like the nobility claim, it was probably a form of revenge as well. "I'll ruin the reputation of the royal family, my family and cause a ruckus for taking away my freedom and using my life like it's a piece on a chessboard."
Aurora has to take her place to marry the prince so she can prevent the nobility and royals from having karina executed. This was all she could do. Aurora contributed to karina's misery, whether she realized at the time or not, she's guilty of deeply hurting karina. Karina has the final say if she'll forgive aurora after learning aurora's motives. I'm fine with karina saying, "I see, cool story sis, don't talk to me or my wife ever again."
Society can suck it man.