I still can't wrap my mind around how she just went, redid the interior of her room and have good will to the point of calling her 'sister'
The Heroine is, by most accounts, an idiot. She's easily swayed by other people and doesn't seem to like acting like a noble. But she's not evil; she doesn't hate our FMC and at most, just disliked her (prior to the story) due to FMC being an antisocial weirdo. FMC is nice but it's clear from the flashbacks that she absolutely sucks at talking with other nobles, enough that she's ridiculed due to her behaviour (and the Heroine ends up getting swayed by these rumors too). FMC doesn't do politics and ends up paying the price.
In politics, you don't have to be the most capable or smartest person.
In politics, you have to be liked. The Heroine is naturally liked due to her personality while FMC isn't (also due to her personality). FMC is too dense that she fails politics.