I disagree with the last part, at least for now. The duke's son realizes she's really a victim, but he also thinks she's too naive to believe him just because he was a bit nice to her, and he also mentions that she thought he was put in a bad position because of her, but it's actually the best scenario for him to recieve her, bc she's the only hope of the empire.
So, I'm not sure if he's helping her just because her power will be the most important soon, but he is considering it while helping her.
Chap 15 goes into his thoughts. He is shocked that she thanked him and his family for helping her. He is shocked that she is worried about how helping her might effect his family's standing. He calls her naïve for thanking him after showing basic kindness.
The text starting on page 12 breaks down what is running thru the Duke's son's mind. Basically the Duke's family should be in trouble for defying the Emperor and helping the FL. But the war will just keep raging on except now the FL has the power to end the war. That gives her a lot of influence, more even than the Emperor.
Continuing on an image shows a flash of her thanking him for taking her in. And there is more text over that image. One text box talks about how he is sure she doesn't know about the power of the ancient gods. The other text box is just him thinking "Never the less to say so is" Which is referring to her saying thank you. He is questioning why she is even thanking them. And his next thoughts provide the answer "The aftermath of the long standing abuse of the emperor. How can she undermine herself without looking at her own value... ...What the hell did the emperor do to her?" There is an image of his hand clenching into a fist, out of what I assume is frustration.
Basically he's kinda thinking what some have been saying in these threads. She has all this power so why doesn't she just take what she want? Why is this woman with access to power strong enough to end the war with the demons thanking me? Why is she even worried about how helping her might ruin my family's standing? Was she abused to the point of loosing all self esteem?
Then he orders a servant to provide her with a good meal, to get her something comfortable to wear, and to make sure her room is nice and warm. He says that so long as he is in charge she will live comfortably.
Re read chapters 10, 13 and 14. Basically there is a tradition where if someone is being abused by their family they can ask the temple to intercede on their behalf. The temple can send a letter on their behalf to the family of their choice asking if they can move in with them. In exchange for asking the demon/god to use his powers to save the kingdom the FL gave the emperor several conditions. One of those conditions were to let the temple ask the Duke if she can stay with them.
The Priest tells the FL that the Duke agreed to let her stay with them after they explained her circumstances. And also that her story made the Duke super pissed off. The Duke was far away so he send his son to pick her up.
The Duke's son has had the situation explained to him. And it made him behave coldly towards the blonde sister. He also thinks multiple times after meeting the FL that it must actually true that she was abused.
The Duke and his son seem like people with morals. She is safe with them. They aren't plotting to get close to her because she has power now.