There's a side chapter in the novel from the father's POV where we learn he's fully aware of Serena's dark guild work and thinks her personality/mindset are exactly what a ducal heir needs. He doesn't do anything about the mother/daughter relationship because he views them as being from different worlds (the mother was raised in a glass greenhouse without any knowledge of the darker parts of society) and knows that Serena just doesn't care. He's super supportive of her, though.
It's the second side story. The title is roughly "the unseen depths of a father's heart"
I really hope they adapt it, but they've already altered some stuff, so idk. In the novel, Tigur is more of a background character and Einrich wasn't constantly trying to get with Serena. There's just this throwaway line when he's talking to his mother about how he'd sooner get engaged to Serena than the sister, and his mother says that they chose the sister b/c she's easier to control/manipulate.
About half is translated on novelupdates, and you can also mtl the raw: https://ncode.syosetu.com/n6845hg/
her family makes me sick!! how can a mother think that way towards her daughter and treat her like that? even the father, he knows the situation yet he’s barely in the picture ☠