obtained the father's blood (the royal family didn't have normal breed process) and successfully conceived, but in order to get Empress' seat, she forced a contract/seal so the royal can't reach the twins unless they agree with her or she died. The mother herself is human (adopted daughter of Viscount? I think, or something like that) which is why people thinks the twins' are 'polluted' since the royal isn't normal human. But in reality, the mother is half/quarter of another royal beings (forgot what) but no one know. The fox is supposed to be the twins' guardian but cant reach them because the same reason (generally).