Responses

Both the father and grandpa believed all the rumors about Arke, she was so scared of everything that never opened up to then. In her mind, they had no faith in her words (which is true, as they proved they don't trust her).
MC had no prejudice against her sister, so it was easier for her to see the whole picture.
Just simple question of wtf was dad and grandpa doing? I guess I'm being a bit judgy but the twin sounds like she's so isolated and nobody to lean on that she idolized and fell in love with that bastard, cheating prince.
I guess they really weren't politically savvy which is hella ironic coz it takes an estranged daughter who is all but a country bumpkin in terms of political know hows yet she fixes the issue with pure indignant fury and common sense.
(ノಠ益ಠ)ノ彡┻━┻