She's not really his sister. Its more of a marketing thing to simplify things in a similar to how X-23 is considered Wolverine's clone although not technically a clone. Nova belongs to an alien race known as Mummadrai that exists as a parasitic life form of pure psionic energy on the astral plane. They require a host in order to become corporeal and Nova used Xavier's genetic material to create a body for herself while he was still in his mother's womb. She shares his DNA which is why people consider her his sister but the Mummudrai are not human in nature