It would be a "unexcepted/less obvious" twist, if Lucius turned out to NOT be Fernan, but his sibling, whose achievement were all given to the other as Lucius is a illegitimate child, or something like this.
Author also has other shoujo/josei WN that got published.
https://www.novelupdates.com/nauthor/tsukigami-saki/
Her latest published WN:
https://www.novelupdates.com/series/i-dont-want-to-become-a-villainess-so-i-aim-at-becoming-a-perfect-lady-together-with-the-prince/
Being queen of one country and getting married to a neighboring country's crown prince doesn't make sense (unless they're doing a austro-hungarian empire sorta thing, but even that had a bunch of factors concerning its founding)
Lesse..... same author as two novel i like.... outaishi and kampeki reijo........ from ch. 1 the story leans more towards outaishi eh... w/e followed.....