Tanith Lee 
Red Unicorn 

The enchanting, magical sequel to The Black Unicorn and The Gold Unicorn! After several years traveling, Tanaquil – a sorceress like her mother with the ability to mend – returns home only to discover that her true love has been betrothed to her sister, Empress Lizra. Broken-hearted and jealous of her sister’s happiness, Tanaquil is lured by a red unicorn into a mirror world where she encounters Tanakil, a diabolical version of herself. She discovers also several new powers. Powers she will need to foil Tanakil’s sinister plot of revenge.