Assuming it follows the novel...
It turns out that FL and her sister are both siscons. Sister spread rumors that FL was incompetent b/c FL is shy, has little magic, and has trouble in social settings, so she wouldn't have to appear at them. FL freezes up over mentions of her sister b/c sis told her to use the situation to her advantage to win over ML. FL felt bad for not following the suggestion, so she's torn b/c she likes both sis and ML a lot.
The misunderstandings kinda push believability, tbh.

There's only a few chap it seems to be the clicker plot of the abused sister deemed as incompetent whils forced to marry a guy no one wants instead of her sis.
I think someone else might be pulling some strings regarding the annulment but I might be wrong. ╮( ̄▽ ̄)╭
I like the mc is the ml and not fl. Even if he doesn't lov her yet he treat her respectfully which is usually not the case from other guys in this genre. They make a cute couple. Hope thes not too much miscommunication. We will see how it goes.