Happiness of the the Holy Maiden’s Fresh Start - Chapter 12

Wait. MC’s sister is being evil now? It’s just that last time we saw her, she was being a white lotus and pretending and stuff. She even still pretended to be nice after the debutante ball. But now she’s openly showing her hostility.
Did the writer forget that the sister is a white lotus and is supposed to be still pretending? She’s supposed to not know that MC has changed (a lot) and treating her like a naive fool. Because even when she thought MC changed she was still reasoning to herself that it’s just momentary and MC will go back to being stupid and a fool. But now the sister is stealing stuff from MC and treating her like an equal competitor.
Did the writer forget the sister’s character viewpoint? I think they did.