House of Fate by Barbara Ann Wright

Judit has one duty: to guard the chosen one, he who will unite two warring, star-spanning houses in marriage. Simple, if she wasn't already in love with the bride-to-be. As far as anyone knows, Annika has been raised to be the perfect bride and future matriarch. Secretly, she's an assassin ordered to usurp the chosen one's mind and kill anyone who gets in her way. When the political landscape shifts, murders and abductions threaten to tear the galaxy apart. Judit and Annika race to uncover the source of the strife. It must be someone powerful and bold enough to risk throwing whole star systems into ruin, someone who could change destinies and bring two lovers together, if they survive.