House of Scorpio (1975)

The book is set in a land called Zodiaca. Races are determined by astrological sign and certain elements can't intermarry. The book includes six complete romances, all with a Gothic feel similar to that of Victoria Holt, but set in the fantasy world of Zodiaca which has futuristic elements but isn't necessarily set in the future... The romances are about six sisters, separated at birth. The final mystery wraps up the story neatly. The real attraction of this book is that it creates a wonderful world, down to its last detail: clothes, jewelry, houses, lands are described painstakingly but despite this, the book never gets mired down in details. This book is out of print and very hard to find, so if you have a copy, hold on tight to it!