Like I absolutely agree it's kinda overly obvious
Yeah, it's not like it's that much of a hidden fact in the novel either (before the official reveal), for the readers I mean. I think the novel still did it better with us really gettting to hear Annabelle's thoughts and feelings on things, which also helps understanding the level of abuse she has gone through.
Spoiler please. How did she find out? Backstory reveal! (ノ≧∇≦)ノ
I really recommend reading the novel! I'll share a link to where I've been reading it, and if you want to know what happens in what chapters, Novel Updates spoiler forum has nice summaries for all that ^^
https://www.webnovelpub.pro/novel/i-quit-being-the-male-leads-rival-12040103
Clearly the red haired woman stole Fl as a baby. Shes also clearly the pink haired boys missing sister.