The Snow Queen — Joan D. Vinge

The Winter colonists have ruled Tiamat for 150 years, slaughtering the gentle sea mers in trade for off-world wealth. But soon the gate to the galactic Hegemony will close, Tiamat will be isolated, and the 150-year reign of the Summer primitives will begin. Unless...
ARIENRHOD, the ageless, corrupt Snow Queen, can commit a genocidal crime-and destroy destiny... unless SPARKS DAWNTREADER, the Snow Queen's companion, can survive sea and city, palace and slums--and find destiny...unless HEGEMONY COMMANDER JERUSHA PALATHION, the Snow Queen's victim, can find one ally on Tiamat--and change destiny...And unless MOON SUMMER, a young mystic, can break a conspiracy that spans space--and control destiny. Because Moon is the Snow Queen's lost weapon. The Snow Queen's lost rival. The Snow Queen's lost nemesis.
The Snow Queen's lost soul. Moon is the Snow Queen's clone...