The Romance of Atlantis — Taylor Caldwell

The magnificent, sensuous Empress of Atlantis expected the foreign ambassadors. She even knew their mission. If she would wed their Emperor, Atlantis would be protected from invasion.
She loathed the man, but had to reckon with his enormous power. So she devised a simple plan. She would give him her lovely and virginal sister in marriage. Since only death could take her from her throne, she would never be forced to surrender her beloved Atlantis in her lifetime!
But there was one thing her hated suitor wanted even more than Atlantis. One thing he desired with a passion and was determined to possess -- herself.