The most probable reason why they were shitty toward her or didn't pay attention to her was because she didn't fight back against the abusers and tell on them. (The elder hinted at her not using her head previously.)
Like, even if she was the child of a prostitute, because she was taken in and kept the name it means that commoners and lower nobles shouldn't be able to touch her at all.
Most likely she isolated herself from fear of being rejected and many other lies that the other lady's faction made her believe.
The other lady most likely did some subterfuge to hide her misdeeds against the MC... it is quite easy to influence a child who has noone into believing lies. I.e. she is hated, she doesn't have rights, she's a whore's child and such things... especially since the maids were under that lady's control. (Evidence is with how they treat her as a whole.)
Since if the lady gets rid of the MC or control her, then they can get the rights to the management of the estate which is a great deal of power.
This is why in the first chapter, the MC decided to speak out and tell on them in a public event no less. If she needed to do that, then it is highly likely that most weren't in the know.
It's actually pretty easy to hide such stuff since they controlled the maids which means all throughout her childhood, the maids and the lady had their hands on her. (Except maybe for her nanny and personal maid.) Also her personal maid can't really talk directly to the family since low ranking servants probably can't go up to the masters of the house and she looks like she's the same age as MC, so they probably know just as much as each other. And the nanny could only take care of her when she was just nursing.
Another thing is she was already quite depressed when she got older and capable of thinking.
Depression is serious shit, even if she wanted help she wouldn't even bother at that point... she even pushed away the only kind soul near her. And I doubt depression has been researched much in that world since even in ours it's just barely getting the attention it needs. And I heard severe cases can cause hallucinations or something, she's become so mentally unhealthy from the early abuse that she could've created a more sinister version of her family in her head and forgetting any possible care she's received.
Tldr: Family did not know anything and thought she was normally like that because the abuse was hidden and she's been gaslit into believing that her family doesn't and wouldn't care.
I'll continue to read to see if it really is like my assumption or it turns out to be flawed writing.