This is just so sad.
Honestly idk what I would do in that situation. She can't even split up, she can't complain about it without her reputation taking a nosedive, and she'll always be seen as the inferior sister.
Sadly, her best bet would be to either find another fiance who is a higher standing than Soleil (which I'm assuming is what'll happen) or to say "screw it" and book it to the middle of nowhere so she can start fresh (but she likely wouldn't know how to function outside of noble society, so she wouldn't last long without some solid help).