You are the spoilt princess of the kingdom of Agria, which gets it money from the sale of it's biggest export, milk.
However, a plague long ago killed off all the cows in the kingdom, so the king turned to the Order of Magi, who turned the slaves of the kingdom into Bovarians.
Female slaves grew more placcid and grew another set of breasts, all of which filled with huge amounts of milk. They became extremely fertile, any sex would make them pregnant. They also became highly aroused, milking them would make them orgasm.
The male slaves turned into bull men, with large penises and immense strength, and enough semen to cover a human woman completely.
The slave race soon became completely dominated, but the curse had one side effect. Any human who had sexual contact with the Bovarians would become one, forever.
Several generations passed and then you, the crown princess were born, and several years later your younger sister was born.