Your right but I dont think she’s supposed to be an adult, it said that she died as a highschool girl and her furthest premonition we see is when she’s 12 years old if I recall correctly so I think she just didn’t think that far ahead and thought more emotionally than logically :3
But she said that there will be war because of the power struggle, then it means that she could she more events further than just 12 years old. How can she be so sure that it was because of this and why nobody stop it when their parents are still there. I don't think just some tradition can have that much impact though, since they are pretty much OP and even the king will have them if they really try.
I think the parents probably die. That’s why they’re still so young when they were talking about her not being a good successor. And the war is likely the fault of a power struggle by outsiders who want to make the heir their puppet. One party backs the current heir, and one backs the one with magic and it leads to a political struggle which blows up into a civil war in their territory (likely not the whole country, but a dukes territory is probably huge)
She’s also probably terrified and thinks she can’t tell her family, or she’s tried and the magic is stopping her (depends on how the author decides to go). She’s now awakened a new power she didn’t know about, so it’s either a new variable, or she’d never seen it in her visions so she didn’t know about it. But she was told never to tell anyone and is now even more scared about what is going on.
Being a teenager and thrown into another world after death may be a normal thing for these stories, but is probably terrifying and traumatic if it were to actually happen. Telling your new parents that you are different is probably scary, especially when you have visions of a future war.
... I'm just confused how the hell did she come up with the idea that it will be better if she leave the house. If the reason is the power struggle, it can only apply to two different parties though. I mean both siblings have the same parents, plus the it is a loving family. I understand that she didn't wish to tell her parents maybe because she's afraid of their reaction? Or just to make some spice in the story or something? Anyways, instead of understanding her family better she's just too focused of a plan she believes is the best, she didn't try to put her parents or brother's shoes to her. Did she not realized that her family will be very devastated if she left? Forget about power struggle or war, I bet her family willbe the one to start the war instead since they will likely think that she was kidnapped.