They didn’t outright said it but yes, most probably switched and Sera's soul resting in MC's body after being heavily affected by black magic.
It's just the mc's speculation: she thinks the first heart attack might have killed the original Sera, and that her twin's death was the reason why she, the mc, felt the attack too, to the point she even got "pulled" to her fill her twin's recently emptied body... But she's unsure if that's really how it went (and we all know it'll turn being that the twins unconsciously switched places in order to avoid the curse, since the curse was weakened bc the targeted soul was no longer the one occupying the targeted body.
Hopefully the twin survive and have a happy ending
I'm a bit confused. I felt like there's scene where sera twin is in camatose in a hospital bed? She's really dead or they swapped bodies at the time she have heart attack?