Tanith Lee 
Red Unicorn 

The exciting sequel to Black Unicorn and Gold Unicorn!The young wanderer Tanaquil can mend anything that is broken — except her own heart. With the engagement of her beloved Honj to her sister, Empress Lizra, she returns home to her sorcerous mother — and her mother’s new lover, the magician Worabex.Caught up in their combined magic, Tanaquil and her mischievous familiar — a literal pet peeve — find themselves in a parallel world where she meets Tanakil, a mirror-image princess with murder on her mind.