MC is basically on 'lol, eff this whole thing and eff you all' mode right now and it's great that she is being smart and cautious about fighting back, but still fights back a bit.
@Ceereus
She's a commoner, the duke had a commoner impersonate his daughter so he could have political power. They're not biologically related, she's just a body double living in place of the real one that died.
@Cae
Did you even read the first chapter? She's biologically related to him, she's his bastard. That's why she has green hair like the rest of the family. The Duke just never told her until she got married off.
@tralxus if you paid attention to the chapter, the black knight
appears to have an ulterior motive. The text suggests that they have some form of chronic illness or condition for which there is no readily available treatment—aside from, for whatever reason, the tassel that the MC gives him. That's why he wants the excuse to keep talking to her.