Yes the real lucia is the one they adopted and she was the one that grew up spoiled....the real lucia wasn't kidnapped, some incident happened outside and she got injured or something some dude found the real lucia helped her and she fell in love with him and decided to stay with him...thats when the fake Lucia came into this world she was drowning or something and noah saved her...real Lucia's brother was looking for real lucia and ended up finding the mc...mc did told him that she isn't their sister but someone else but the brother thought she has some head injuries or she lost her memory so he didn't believe her...so inshort the real lucia is alive and well in the same world mc is in but she just decided that she would live with the dude who helped her that's why she send the letter home telling them that she wouldn't come back and that's why the second bro thought that the mc was fake... I don't know if you want it or not but here's the summary
So when exactly did MC, fake Lucia enter the mansion? Does she know she’s not the real Lucia? Was real Lucia the one who was adopted as a child and grew up spoiled? Was real lucia kidnapped, and then they found fake Lucia? And only now has the hawk been able to deliver the letter? Nvm I think I get it, tell me if I’m right