Jesus, this was beyond the level of trashy family, I think the TL was abit mixed up because I couldn't fllow it very well but the MC father felt like he was doing his outmost best to make the good daughter get married to the worst noble he could find.
From what I've understood he simply let the bitch sister lies pass because the prince was stil lconfused after waking up.
The potion part was weird, MC said it was made to lose memories but then the pricne wanted it because it would have made him recover his true memories, that is why I think there was some mistranslation.
The bitch sister can't be forgiven, even if what she said about a relationship with the engaged prince from before the incident is true she lied to force him to hate his sister, and even proclaimed the sister tried to poison her, but the sister simply forgiven everything and let it pass, I hate wimp MC so this is enough for me.
I mean, she knew her sister was spineless and wouldn't stand for herself, just ask for the engagement to be switched.