
According to some spoilers. Her being the biological daughter doesn't really do much in the story's plot. The father and brother found out about it but FL tell them not to tell her sister and not to change the treatment they both receiving (you know the FL being on the more unfavored side. And the sister being slightly doted on.) Plus, the father said, when he found out FL is the daughter of the duchess he thinks along the line of, I should've known, coz your smile looks exactly as my wife. Or something like that. In the end, it didn't really affect the story that much and they both regret their actions (bro and father) but FL just no longer care.
And oh by the ways, if I remembered it correctly, she was the Original villainess in the story. The novel was actually their first timeline. She just went back again to that timeline but thinks that she was reincarnated in a novel.

Thanks! Kind of knew this was her original character or this was her real story and she was just imagining her being in sister's place and feeling what she felt like some distorted memories but I think she did went through reincarnation/body possession(in spirit state according to lore) then time traveled back to her original world because how does she know mana techniques of Murim/jianghu? Maybe side effects of turning back time?
I don't know if anyone notice but I love how the former duchess managed to protect FL even without knowing she's her real child.