Okay in why the brothers are acting since.
A lot of Media portrays abuse wrong. In a way that the Abuser hates the victim. I get that for some who hasn't been abused it might feel that way but it's incorrect.
Most Abuser love their victims but they treat them in a toxic manner due to something personally wrong with them (anger issues, narcissism, antisocial behavior, the list goes on.). Abuse has a lot more to do with manipulation, control, and dependency than hatred.
In Penelope case the power dynamic between her and the Eckhart family is way to high. Penelope is a peasant girl who do the only reason she has any status is she's the dukes adopted daughter cause his real daughter went missing. The Eckhart family has complete control over her life. With losing the first daughter that probably made the sons and father keep Penelope at arm's length emotionally never truly attaching to her as an individual.
Now the reason why the brothers are acting nice is 2 reasons.
1) she was just attack by the prince. As a ducal family the Eckhart are only 2nd to the Royal family. So she currently has a bunch of sympathy. As well they can grandstand against the royals politically. it wouldn't look good if the public learned they abused her.
2) in the early chapters Penelope told the younger brother she was planning on leaving after her coming-of-age ceremony. Which is well within her right to do. This would be a huge end to the abuse for Penelope because she distance herself from them. But remember she acts as the families therapeutic emotional support it would be devastating if she left them on a sour note. They would remember how the real daughter disappeared and have to face those emotions after all these years.
So they need Penelope around for completely selfish reasons. Not because they are genuine.
Remember after revealing she getting stabbed by her maid with needles and being fed spoiled food. The father and older brother acted like she was inconveniencing them cause the maid had high qualifications and it would be a hassle to replace her. While the younger brother wanted her to throw a tantrum despite the fact that she's been doing this for years and he was the one calling her a liar.
Seriously they all sound super toxic and will probably betray her at some point. Especially when the real daughter returns.