Omg, I agree. But honestly, rather than blaming the father, I blame the mother. She's the one who is literally forcing her to become empress, to the point where she'd deceive her by masking her surroundings. The father is an awkward bump, I bet of she saw the man as a Dad than an Emperor, they'd get along swellingly.
I mean I know that it isn't technically as much of the fathers fault as the mother but the thing that bothers me is yes, the mother is using Sienna for her own desires, and she doesn't really treat her father as a father, but also if u really think about it her father has never really tried to get close with her either(at least from what we can tell so far). And the fact that he's trying to get closer with his lost son and he is in fact closer with him now, and although I get that you would want to get close with your lost son, he hasn't tried to get close with his daughter that has been with him for her entire life. Like thats the part that bothers me. Like their lack of relationship isn't only Sienna' s fault, like neither one of them has tried to reach out to one another -._-.
For the father, I believe it's the iron king that's trying to get closer to the father, not the other way around. The iron king tried to fix the "misunderstanding" by explaining it to the MC so that she wouldn't get suspicious of him, but in reality, I have a feeling he's trying to get closer to the father by acting as a "loving son". (Although, at this point, I'm not sure if the iron king really sees him as a father or is just acting in order to get his favor)
I bet the father is one of those "Cold on the outside but warm on the inside" kinda person that won't open up to you without you doing that to him first. MC had never seen her Father as her "Father" but the Emperor. She had never thought of treating that "high" being as a father nor has she ever thought of getting personal with him which is why she kinda froze when the Iron king told her about his business with the Emperor.
Yeah that does makes sense I guess, but for some people it's also like if one side doesn't try neither side will try. And since she grew up being taught that her father was all high and mighty without him like trying to be there for her(maybe idk yet) so I guess like after a long period of time without any contact with him she just didn't see him as a father anymore, just as the emperor and as someone who biologically was her father. And yea, this wasnt entirely his fault also kinda her fault for not trying either. But like what I said earlier, if no one sees one side trying at all, no side will try. And yea I do agree with how you said that he was a "Cold on the outside but warm on the inside" person, it's quite possible.
Honestly, the fact that she was only raised for one purpose, the purpose of being the next heir, only for it to be taken away is sad. She worked so hard since she was born just to be king, and her sole purpose that she was raised on just disappeared. And honestly I know that the brother has his reasons too but like there are other ways to do it other than just stealing it from her. And that bitch of a father don't even get me started on him -_- like your daughter has been with you her entire life and yet you've never shown her any love and affection but the fucking moment your lost son comes back you can have a happy family with him? Like you can just give him the throne after making your daughter work like a slave to get where she currently is??? AND also the way that Khun is technically "two timing" her with the Iron King. Like he's hot and all but like you're just gonna help the Iron King snatch away the only life that she's known since birth while you fall in love with her and make her trust you?? Like bro, you gotta choose a side ;-; Like honestly, she hasn't gotten any love and affection her entire life, her father doesn't seem to give a shit about her and her mother is just using her. Like that's sad, you can start to see that she's starting to feel upset at the fact that the father is treating them so differently too. Like all her life she thought her father was so far away from her but with her brother even though he hasn't been there as long as her he seems closer to their father than her.
(Man, I probably repeated myself so much but you get the point ->-)