
I think it's more like he doesn't care about his sister and his sister is being stupid. He wants a war and it's obvious that his sister would get the short end of the stick (i.e. death) and its intentional. You can see that he doesn't even think about her. Sometimes those who are sent as hostages to others kingdoms/countries aren't sent because they're always precious. Besides, she's a princess and princesses are more disposable than princes. Though the princess should be able to see how this would end for her and for the red haired prince.

I mean I know the concept just I thought he gave > 0 fucks about his sister. Clearly he does not if he is involving her in his schemes. Also I don't understand why she did it if she cares about Diego at all. Like girl you love this man so much you almost murdered the only person who was nice to you and treated you as a friend...so why you trying to hurt/kill him now

Either she's so mentally unstable that she can't see something so obvious or she's downright an idiot. To play devil's advocate though she might think there's no chance of Diego picking her so she's trying to get him even if it's underhanded and he's injured in the process, as long as she gets him. Too bad if her brother got his way he would be dead in the end.
Yes my sister I'm going to start a war while they are holding you as political hostage... um am I wrong or is a political hostage exist to be killed of the other party starts shit? Does he not care about his sister or are they both just idiots?