they are an asshole in truest sense if we see from FL perspective, if you've read the novel the FL practically working her ass off so that she can be a good wife for that bastard of a man, you see her eyes had sleepy bag is because she always studied all day and sheldom taking a rest while she see in the window that girl playing happily with FL mother and father without a care of world, even the servant in their house do a minimum thing for FL with no respect at all while they treated that girl like a princess. This is why the knight is sympathetic with her, it not because the knight loved her but he sees how hard FL study to be a good wife for that man, if I'm remembering it right FL study until her nose bleeding and barely eating.
While the man? He just like an innocent pure type like FL little sister but do you know what this bastard say in his mind? The little sister not suitable to be his wife so reluctantly FL had to be the wife like wtf man, even if you don't like her give her a minimum level of respect cuz she literally forced to work for you, i don't know if this can't be called an asshole.
The little sister is ignorant, she nice at heart but in another word it can be called selfish, she believe everyone can do what they want like her, but if by ignorant it justified her to hurt FL emotionally like that is bullshit (stealing your older sister husband is never a good act)
And the FL all she want is to be loved by somebody, her parents never even pay attention to her since she was a kid, that why when the parents introduced that man as her fiance she happy and devoted as hell cuz she read that a husband will loved her wife dearly when she was still a child, that why she crazy for him at first cuz in her mind he there for her.