
I mean Diana is blind, of course she will hold his arm. She cannot see a thing !!! Of course she will be kind and happy with him, he took her in to help her. Diana tried to clear the misunderstanding with the FL, and she took it as a lie?!
The FL created a whole soap opera in her head. Does not even listen to the ML when he tries to reach out to her. And the will of the father was a wish, not an order or obligation, if he loved Diana he wouldn't marry another girl. FL spins some serious narrative in her head and goes a little to far.
If I learned something from mangas and chinese webtoon is this - couples remember to communicate. Communication will prevent years of wasted time.

I will not tell him that it was a mistake because he will kill me. Good idea. Let him find out on his own that you played with him, certainly the reaction will be more understanding.
I mean how long does she think that it will last? When he asks for marriage and she says no? At some point he will find out that you are craving that double-faced blondie and he will be more angry. Just like now. ╮( ̄▽ ̄)╭

Is there any explanation for it in the novel? Pretty please tell me if you know.

Actully no, from the last light novels look like the problem was the mother bloodline.
Queen Lacey La More, a former Duchess of La More become a Lycanthrop almost immortal, in Volume 6 try kidnaps the Duke Daughters but leave behind Melida & Mule because one "even if from the noblest bloodline she was tainted from a vampire" & the other because was not a real member of the La More family.
... for her to actually become sadistic. xD
Same here (๑•ㅂ•)و✧