I'm very curious about the father Adonis. The last chapter indicates he was just negligent and not outright cruel like the step- mother and daughter. In this chapter, it seems Adonis had no idea where the previous head butler went. I think this means the butler was dismissed by the step mom, and Adonis had no idea.
In his position, he cannot avoid responsibility. He allowed his domain, which is a frontier of the kingdom and needs to be well maintained to stave off attacks, to be overrun by corruption and worse. Being a super meathead isn't an excuse. He apparently noticed that the competent head butler disappeared 5 years ago! It was obvious to him that things were happening behind his back (I mean you can see his reaction to his wife cheering him on when he went off to fight that he wasn't wrapped under her thumb) but he just kept ignoring the situation. It's definitely unfortunate for him that he was put in a position of responsibility which he couldn't escape from, but the least he could do was ensure that he has people he trusts to manage things properly yet he allowed all of the competent servants to be dismissed by bad actors.
The brat was the main reason why Aria almost died a miserable death in the first place. She knew what she was doing, but just didn't care because she was so jealous of Aria. She doesn't deserve to be executed, but she deserves to be put in prison.
That's incorrect. Her mother is the one who wanted Aria to be dead so that her child would inherit everything.
While the manga depicts her as an independent actor abusing Aria, children take some time to develop independent thoughts (especially ones which may run contrary to their role models) and usually copy people they trust, that is their parents. She was also raised, in her early childhood, by her mother alone as a poor commoner, and so the environment and her mother both enabled a cutthroat thinking instead of any kindness because that was the only way to survive.
Of course, at this point, she's pretty rotten and I can't condone anything she's done, but I can at least understand it. Where you have poverty, you have crime, by necessity. (I would be much less forgiving for those born in wealth who still practice crime.)
Either way, her future is bleak; prison for a young child is absurd, but freedom is just her becoming an orphan with no support and destined for the slums. So if she does avoid execution or prison, then the best for her is to be placed somewhere to learn a trade and at least become a productive member of society.
