The fact that she doesn't reject him right there, she either returns to the Main house, or leave the Annex for disobeying him. If she refuses, she has nowhere to go. Honestly she should leave, but it's understandable that she would see both as bad options.
FYI, she's Rua's older cousin, and the one who trained MC to be the badass that he is. She even calls it out in the first chapter how him breaking away was gonna cause a lot of chaos. She might even be a greater threat than the Pig.
Check the first couple chapters. She's horrendously spoiled by her Grandfather, a narcissist, and basically rich enough to get away with murder. She is beyond hope.
Makes me glad our MC is making it his mission to deal as much psychological damage to her as possible.
The biggest problem is not that Rua is spoiled and unwilling to learn, it's that she has her grandfather (and his company) enabling her at every step. She has no incentive to change if Gramps is just gonna bend to her every whim.
Okay, first, as an older sibling I felt that way too hard.
Second, I didn't need any more reasons to hate Mia's parents, but here we are.
Third, I cannot wait for the sweet, sweet revenge that's coming. Mia has far too many allies for it not to happen.
Remember, they treated all she did like it was burden, and never actually noticed how much Philia was doing to defend the nation. The Idiot Prince was too stupid to look any further into the situation. All he wanted was to make Philia's sister his fiance.
To be fair, this is much worse than death. Now they have the king on magical speed dial, and he knows he can't fight back, both physically and politically. He now has to live knowing this overpowered family can summon him for a beating at anytime.
Honestly, I'm not surprised.
Mia has been basically spoiled her whole life, and was likely blind to everything. Hell, I'm certain everyone already knew how much she adored her sister and went out of their way to prevent her from seeing all the abuse Philia went through just to keep her happy...