I respect your point but the thing is, just because Aquila has a reputation, doesn't mean that he's the only one she should blame. They are a very powerful family, there are a lot of nobles that may want to harm them for having said power.
Also, there's just not enough evidence to immediately accuse Aquila. Suspecting and believing that someone did the crime are two different things. He's a cruel jerk, I admit that. But that doesn't mean that he's the only evil being around.
I've only read until chapter 10, because honestly this comic isn't exactly my cup of tea, but here's how I see it:
It makes sense.
I think lots of strong-headed characters tend to have a specific flaw, mostly anger.
anger makes people blind to other possibilities of course, but it's not even just the anger that actually makes her think that it's Aquila, it's most likely the fact that she just can't think of anyone else with enough of a dislike against her family enough to actually kill her father.
Of course I agree that he's not the only threat to her family but so far, ever since her brother came back with a crippled hand, her dad has been sending claims and requests for investigation on Aquila, in her mind, this probably got around to him and now he wants to protect his reputation and the claims will probably make their way to the public at some point and it could make his reputation worse therefore worsening his chances at becoming king.
Lamia so far does not know Aquila well enough to know for sure if it was him, or really an accident. she's sure it's him because he's been the main tormentor throughout most of the latter half of her life. It makes sense that she'd almost blindly assume it's him. She knows that he's not the only evil, but for years he's caused her family pain. And to her, all the dots connect so to such a head-strong character like her, it's enough evidence to go barrelling into Aquila's mansion demanding an explanation.
I guess the point is that she's not exactly perfect, but the conclusion isn't purely due to her bull-headedness, but it's probably due to a lack of knowledge.
She even ignores her brother who tries to tell her that it wasn't Aquila specifically because it doesn't match how he acts, but she thinks that he is just too scared to go after Aquila and that automatically makes his opinions on the duke invalid because it would be easier to just not listen to his opinions on subjects that are traumatising for him.
You could argue that this is what makes her a bad character, but even this behaviour makes sense. She lets him go to the capital against her judgement because he seems so desperate to do so, but after ages he comes back with a crippled hand, and a haunted look in his eyes and he just can't act the same as he used to. That would definitely cause her to become afraid of him disobaying her and that would also mean that she's less likely to listen to his opinons because the last time she did that, he had his hand be rendered effectively useless.
The story is slow and confusing but it definitely is interesting. I want to know their past.
Also, I find the FL a little bit annoying. She looks and acts like a smart girl but the way she just jumps to conclusions and blames all the sins on the 2nd prince is dumb. She doesn't even try to find conclusive evidence and just assume things because the 2nd prince is "the only one who can do this".