Theory

I don't wanna justify her actions, because acting on feelings especially in such a precarious situation (in a completely different country, no family, etc) is never a good idea, but since they've had such little contact, I believe Mia's feelings are being forced by the book. That or she's brain dead cuz you never go for someone already married.

Also, regarding the imperial brother, I think he was either also reincarnated, or the novel itself is a different book altogether. Leana has no solid proof that this book is the one she knew, it could have been a spin off or fan fiction that she got reincarnated into. It would explain all the differences, as well as the presence of the brother and the tension between the church and the imperial family