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If I'm remembering correctly
Mc is the real daughter, Swan never was the real daughter. The mother had the power of foresight and knew that Mc would be kidnapped.
This is the part I'm too sure on
Either the mother put her powers in the necklace and knew Swan would be the 1st one to touch it and be the "fake" for Mc to grow up safe
Or
She didn't know Swan would be the 1st one to touch the necklace. Since her powers and memories? Would protect Mc until a certain point and hopefully by then the father would have found her already.
The necklace had some kind of holy power and a kind of imprint. Since Swan was the first one to touch it as a infant. It imprinted on Swan, extending her lifespan & changing her looks. So Swan never was the real daughter, a decoy made by mistake