Even in the flashback it shows just swan as a baby. It is strange the both shown up on the same day at the orphanage
As a novel reader spoiler alert unfortunately they're not twins
The locket that was supposed to be for mc was touched by swan first and changed her appearance that resemblance the duke and duch, cuz the duchess put a magic in the locket to protect their daughter but due to swan touching it (suppose to die early like as 2yrs old) it gives her 10 extra years to live so yea the real daughter is our brown haired mc
I didn't ask for spoiler...
I thought this at first too, but then I leaned more on the side of maybe Swan wasn't really the daughter because these type of stories always leave it being the "fake" as the real unless pointed out otherwise. Oh, I see someone threw a spoiler down.
This is not a spoiler but just my theory. I think lily and swan are actually twins. That explained why lily has a bit resemblence to the duke but it just not prominent since she's still young