So the brother wants to become emperor to survive, but according to sienna's visions, he ends up dying rather young anyway. Perhaps some years later than if he didn't take the throne, but still. These siblings need to be honest with each other imo. Neither of them hold any ill will, but instead have fundamental misunderstandings in their view of the other.
Sienna knows basically zilch about her brother past the visions because she's been disinterested in him (and her mother likely tried to keep her away too?), and he doesn't know that sienna is just as much of a victim of her mother as he is, just in a different way. I know the way the brother patronises and looks down on sienna is irritating, but remember that she was guilty of doing the same to him before she got those dreams. Both of them are victims of ignorance, and they may sooner or later get the chance to rectify that problem.
I don't care about the romance at all (too instalove-y for me), but i'm definitely interested in seeing the plot unfold. Sienna is a wonderful, level-headed protagonist, as always.