
He betrayed the country by betraying her but his goal was to defeat the other kingdom and seize power for their own under any circumstances, even if it meant sacrificing his own family. That's the only difference between the brother and the FL. The main girl thinks marriage is going to be what saves the kingdom, and from the brother's perspective and the information he's given, he's right to assume she is naive to how most hierarchial societies work. Secure an hier if you can't do that the legacy is dead. This isn't to justify the brother, but in terms of the FL, the desire to protect her kingdom comes before anything else, so siding with her brother despite her distrust of him was the only way she could've gone.

At the end of the day no matter who started the war, the cost of it was the people she loved and the one that swung the sword to end them was her husband. Logical thought is not going to play a part when it comes to grief. She is right to have some hesitancy in trusting him. It would be stranger if she fully trusted the man who had burned and slaughtered her kingdom.
She so fucking stupid like s good and whatever yadada but like why are just gonna forgot YOUR brother started the war that’s the only thing that like making me upset yeahhhh the ML killed and slaughtered your people but honestly girl I could care less at the end of the day I PERSONALLY think she really needs to kill her brother because he’s a danger to society. Like you cannot blame HIM when you saw with your own eyes ur brother not only betrayed you but his entry country, HE due to his choices killed his own people they all just didn’t die by his