So her kingdom broke an agreement, which caused the emperor to attack/kidnap her sister in retaliation? Since it's a tripartite agreement and their kingdom is so small, I can only imagine they did it together with the other country, or at that other country's instigation.
I'm reserving judgement on the emperor for now. He made a good impression on me in the first chapter, and it sounds like he had just cause for being pissed with the MC's country. Plus he was clearly a likeable character in the book.
I really like the concept of being thrown into not a book's storyline, but the ridiculously dark past that leads up to it. Authors rarely hold back with tragic pasts, so talk about being screwed 🤣