That would destroy her sister's life, maxi can return to anatol but her sister would stay with the father, she's unmarried yet. And if she hides in anatol the father can do what riftan did to her maxi back and wage war. It will be an endless cycle, that's why spoilrr:.
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after she marries the Prince in the time skip the Duke declines because riftan and her become the Duke's enemies

I have a better idea..why won't you call your sister for testimony?? She can literally be the eye witness we have! And that's a easy way too..