it seems like FL is not the real child of Caitlyn because FL's birthday and the Aaron sister's death anniversary (his sister died soon after birth) is on the same day and another coincidence is the FL having the same dark blue eyes as Aaron.

What bothered me the most though was how they handled the twist.

Twists do need foreshadowing for sure. There was good foreshadowing here too that sharp readers could pick up on, like how Annabelle's birthday coincided with the Rainfield family daughter's anniversary, or how the MC didn't look similar at all to the Marquis, her supposed father. They were subtle enough that they could be overlooked, but specific enough that people who noticed it could feel rewarded at the reveal.

But the author then decides to dedicate an entire scene to where Marilyn talks to the MC about how she gave birth at the same time and in the same room as the MC's fake mother, and how the nurse in charge of them left the area to go live in a expensive house somewhere far away immediately after.

That was pretty un-subtle foreshadowing right there. But in my opinion, it's still within bounds of a decent novel. And yet, perhaps it wasn't enough, because the author decides to add another scene where this time a traitor knight from the ML's family steals a jewel from Marilyn, just to show that Marilyn swears a lot like the MC.