SOLVED. sci-fi/fantasy book with wizards crossing universe [s]

[Katia]

Hi, this is a YA or adult fantasy book that I read about 8 years ago. It is told from the perspective of many characters, two of which are a young woman and her younger brother, Nick. There is a scene at the beginning where they are at a (possibly sci-fi writers) convention, in the middle of cars, snapping their fingers and singing, spell-casting without knowing it. They are actually the heirs to a galactic empire elsewhere, but they are hidden on Earth, and don't know who they really are. The woman also becomes a powerful mage.

I think the cover had an aggressive unicorn on the front, and there were a bunch of people and creatures coming through a doorway.

[Sue]

This is 'Deep Secret' by Diana Wynne Jones.There is a sequel starring Nick, called 'The Merlin Conspiracy'.
"All over the universe, Magids are at work to maintain the balance of magic, using their own talents to push the right people into doing the right things at the right time. And on Earth, the Magids are working hard to coax the world into its rightful place Ayewards, towards magic. Rupert Venables has been the junior Magid here for only two years when his sponsor dies; it's up to him to find a replacement. Trouble is, the most promising on his list of five names, Maree Mallory, doesn't want anything to do with Rupert Venables. And while the junior Magid is trying to track down the other four, the fatelines are becoming dangerously entangled on more than one world and magic starts getting out of hand . . ."

[Katia]

Yay, thank you!