She's not, if the girl she saw really was her age that would've meant the duke had been cheating on his late wife.
She's not, it has been stated early in the story that she was adopted. She was the product of her mother's prev marriage.
Also, the girl Raina(?) shouldn't be someone the 'duke cheated' w his wife but the daughter of the first duchess. Since she look like the duke, maybe their legitimate daughter the duchess carried before her accident.
He's not cheating. Here's some spoiler, the previous duchess run away while carrying the baby. The duchess did not have any carriage accidents
Why did she ran away
It has something to do with her tribe and the reason she marry the duke. I don't remember what tribe she was as i read the spoiler somewhere here in the comment.
Anyway so she was a part of cult that can swipe soul or something. And this cult has been trying to infiltrate the dukedom so they send her to marry the duke. If they have a child, then the child will be killed and and one of their tribe people soul will go into the body.
She marry the duke with the intention to betray him in the first place but she fell in love w the duke and love her son dearly. The son, lud... Ludvine? What was his name again? Anyway the son had divine protection so they cannot swipe soul. When the duchess realises she had another child, she ran away to save the new child and the dukedom.
I don't want FL to be the real daughter of the duke, because if she is then what happens to ravi? that'll be sad for him he'll really feel isolated and betrayed