The Future Falls by Tanya Huff

When Charlotte Gale's aunt warns their magical family of an approaching asteroid, they scramble to keep humanity from going the way of the dinosaurs. Although between Charlie's complicated relationship with sorcerer Jack, her cousin Allie's hormones, the Courts having way too much fun at the end of days, and Jack's sudden desire to sacrifice himself for the good of the many, Charlie's fairly certain that the asteroid is the least of her problems. But together there isn't anything the Gales can't deal with—except possibly each other.