@Mathiason none of that matters if her father cared about her well being more than the well being of the family prestige.
he couldn't read her face or emotions? she is his daughter if I had a daughter who came to me and said she wanted to join a cult even if I KNEW 100% that was what she wanted I would argue against her, ask her to stop and rethink everything. not a one-sentence "YOU WANT TO JOIN A CULT!?". I would reason with her try to see whats going on with her, instead of letting her go through an incredibly damaging process that, in the manga's case, surely leads to death.
look at her face on page 17 and page 22 of the first chapter, if her father couldn't see through his daughter of many years' words then he is not a father who cares about his daughter.
truly depressed/suicidal people don't talk with their friends and family about the things they are going through, you wake up one day to find out they are gone.
her telling her father after all those years of HIM urging her to be the prince's fiance that she is fine with it and instead she is going to just throw herself off a cliff IS a cry for help, you see the desperation in her face, but you don't see any sadness in his.
we will just have to disagree on the matter.
to me, the prince is maliciously stupid, maybe he didn't mean to be malicious but his ignorance about how people are is one of the causes of her suicide by dragon.
and to me, her father is just a bastard who only thinks about the family prestige instead of the individual members in his family. not what's good for his daughters but what's good for the family image. otherwise, he would have sat down and had a candid talk with both his daughters individually.
and going by her little smile when her sister decided to shove off a cliff, I doubt Luna gave a damn about her sisters' feelings towards the marriage and was just saying that shit to spite her, or said it to manipulate her sister into giving up the marriage. I have a younger brother who growing up did this exact thing to my older brother (younger was selfish and manipulative and older was more timid and withdrawn) my younger brother would do whatever he needed to do to make a situation go in his favor. these types of people only care about themselves. "what can you do for me" "how does doing that benefit me". the sister deciding "if I can't marry him I would rather die" and then refusing to eat is indicative of this selfish behavior.