
originally she only let him get attached to her in hopes that it would prevent her sister's death later, since it was that attachment that led to big sis death in the original story. eventually she ended up loving him though.
the reason she's leaving now is because of what happened with her sister in the past, the reason big sis was fired as a member of the knights, it would cause them problems to be involved with the royals. so she's taking her sister and making a run for it!

It is always bc they "think" abt ML future with "original" FL - that is so naive and stupid, at the same time a lil bit selfish. But with the explanation at the end of this topic, if she is not running so the duke will be with "FL" but to again survive and keep her sister safe, then that is more reasonable than just as often this type of mangas give as an excuse "bUt U wIlL fInD OrIGiNal FL~"
Please tell me she doesn't do that stupid shit of leaving him ╥﹏╥