I think mc caused the curse because in chapter 3 after she found out she was betrayed and confronted theo the king she yelled "the last bloodline of Blanche, I curse you"
Also in chapter 34 they say the curse started when Atalenta became a grand duchy and in chapter 4 we are told the aunt of kind Theodore married Atalenta when he was just a duke before he became a grand duchy so that might be important.
I think mc caused the curse because in chapter 3 after she found out she was betrayed and confronted theo the king she yelled "the last bloodline of Blanche, I curse you"