It's because for years they treated her badly and are only changing now because Penelope is no longer acting like the violent girl she originally was since she is now replaced by the MC. Like the younger brother literally framed her for stealing the necklace of Yvonne and the father didn't believe her pleas of innocence and immediately made her guilty and punished her for it. The brothers basically thought of her as a replacement for Yvonne and because they hated that they treated her badly, shunned her, and ignored the bad treatment she was receiving from the servants. The father never tried to fix the son's misconceptions and just let them treat her poorly, likewise probably knowing that Penelope was being mistreated by the servants as well. Even if the family didn't know the true extent of the mistreatment of the servants, they would have at least had an inkling, and it didn't help that they did nothing to try and get them to treat her better.
To be fair Penelope certainly didn't help things since her original character was prone to fits of violence and temper tantrums so no wonder the brothers and father never warmed up to her nor tried to step in, because they believed it to be futile as well as probably disgusted by the audacity of someone who wasn't even truly related to them by blood. HOWEVER, that does not excuse the fact that it was because of the mistreatment and hatred in the first place that Penelope became so violent, since she believed she had no other option, and she wanted to get her suffering known somehow, even if it wasn't the right way to do it nor did it actually help her.
In any case, the brothers and father are dicks. Like the OG Penelope deserves better. Even if the younger brother regretted framing the OG Penelope for theft(which seems to be hinted in the manhwa), he never tries to make up for it, nor apologize or anything. And ONLY because the original MC is level headed and not willing to act aggressively, only now are the brothers and father like "oh" and are starting to understand how they originally treated her was bad. Like I absolutely hate the fact that it is because she is reincarnated that only now they are seeing Penelope in a new light and want to treat her better now.
Anyway thanks for reading my TED talk and sorry for the rant I dunno how clear I wrote this I'm still angry at the duke fam.
she got her food poisoned by the servants and never did anything because they didn't give a fuck, also they accused her of staff she never did not to mention what they did to the original Penelope, they killed her ultra cruelly(melted her face, dismembered her e.t.c) and most of the times for no real reason. Original Penelope wasn't bad but was lead to that behavior due to the intense emotional(by the brothers and fathers) and physical (by the servants and knights) abuse she went through just for existing. At a point in the novel it explains that the pink haired trash knew she got abused by the knights but he couldn't care less. If you seriously think that such a behavior deserves forgiveness just because they suddenly liked her passive behavior(since she no longer 'screams' when abused) and decided to treat her more like a human being then there's seriously something really wrong with the way you think
also calling her a "slut", and attacking her in every single chance they find makes them caring? Oh yeah, now she eats normal food, i guess that's 'caring' (but only because the maid cooks them for her, if it was any different she would still eat that). Just because they don't directly grab and beat her to pulp doesn't mean they don't abuse her.
it's weird since they were big dicks to original Penelope but now they act like they care and lil bro saves her from the servant that abuse her despite knowing all about it for a long time and not give a shit ╮( ̄▽ ̄)╭ I don't get how they can act as if they care now after all their dick actions, it sooo weird to me Idk ( ̄∇ ̄")

People acting like they done such bad things.. Maybe its because ive read these stories alot but I dont see anything they done as that bad?? They seem to care for her in a way.