There wasn't any mention of that at all. She is a daughter of the friend of the duke's wife. Her mother seemed to have some sort of power (I'm guessing it's divine power since she was initially living in the temple) but it was gone temporarily and everyone thought it would never go back so she was released by the temple. It was then when she was tricked by the prince (not the crown prince) into her demise. Rudia's/Diena's father is most likely the said evil prince who tricked her mother.
The Duke on the other hand, had a wife he loves deeply, he has 3 children with her. But his wife died and he succumbed to depression thus the usual eyebags and drunk attitude. His wife also told him to take care? Or at least here out her friend's request (fl's mom) so to fulfill his wife's wishes, the Duke searched for FL and made her his daughter in law.
That's what I got from 5 chapters we currently have hehe

Err am i the only one confused here? So diena is the daughter of the duke, so that means she’s siblings with the rest of the boys right? So why daughter-in-law? Isn’t her mother the wife of the duke?