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'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.