Not sure on the first one,
But as far as the this story so far since I haven’t read the novel, she’s is an illegitimate child, probably with a courtesan, but more likely with a maid, I think that’s why she’s treated worse, because some courtesans were nobles, but I don’t remember who her mum was just that the father left her to be raise in that place where she was abused, but Cesar is also illegitimate so I don’t get why she was the only one treated bad, maybe cause she’s a girl…
Anyway lastly it doesn’t show her other self ever having interactions with monsters so if she did have the power before maybe she just never got the chance to use it, because it was her new self’s choice of avoiding the original plot that got her into the forest where she ended up talking to them. It seems she was never allowed to leave the castle so it’s unlikely
To the first question:
She received forms abuse from both. Cesar being the worse one because of his obsession with her. Cesar has actually physically abused her. I’m pretty sure he’s the reason she has the scars on her back and thighs. For the father, I don’t think it’s ever explicitly shown but possibly implied. There was once in a flash back she wanted to go against her father and he gave her a threatening look. I feel like it isn’t so wild to imply that he possibly abused her or got someone else to abuse her.
Even if the father never touched her, he doesn’t care about her. Only how useful she is to marry off. The only other decent person in her family is Enzo and he has the least amount of say being the second brother.

I have so many questions.
First, did the brother whip heror the father?
Second, is she adopted or is she an illegitimate child with a cortesan?
And last, did she always have the power to communicate with monsters or is it because of her new soul?