My theory is that the blonde boy who died early on was actually the duchess and dukes kid and then Yoidel was the child the duchess had with that red haired lover, but they used forbidden magic to transfer the soul of the princess into her which would explain why her hair changed from red to pink (kind of like the soul changing causing the body’s appearance to change). Also them transferring the princesses soul into Yoidel could explain why they sent her to the church, expecting her to have powers and such which would be useful for their plan!
Spoiler
My theory is that the blonde boy who died early on was actually the duchess and dukes kid and then Yoidel was the child the duchess had with that red haired lover, but they used forbidden magic to transfer the soul of the princess into her which would explain why her hair changed from red to pink (kind of like the soul changing causing the body’s appearance to change). Also them transferring the princesses soul into Yoidel could explain why they sent her to the church, expecting her to have powers and such which would be useful for their plan!