Responses

Just a theory. Iirc from the novels,
SPOILER!!!!
(the witch who originally cursed the first head of ML's fam posseses the princess' body. Shes an obssessive little biatch who kept possesing other bodies bec she's connected to the curse. Also i never read anything the children veing brainwashed. The novel has a Good ending
Do not read if you've just started....
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The shackles while easing the pain are a way for the princess to control the members of the family (so a very bad idea beware the doctor is gonna be brainwashed and/or die. As we saw those two children were brainwashed. I think, maybe the fourth princess is a possessor that has been working her way down along the line- probably someone from the time the curse originally started happening. Hopping from wife of the head of the family to wife of the head of the family and gradually letting their blood turn toxic.