Shon'jir (The Faded Sun, #2)

Sten Duncan had saved the lives of the last two of humanity's deadliest enemies, during the takeover of their planet Kesrith. Sten therefore felt responsible for them and for their future -- if any. For though the two mri were brother and sister, they represented different power-castes of their ancient warrior-race. Niun was the last of the bred samurai. Melein, though young, was perforce the last priestess-queen. But struggle and mutual danger had sealed Sten Duncan to their loyalty. As their blood-brother, he would have to help them flee mankind and take the long, long evasion-route across the cosmos to a legendary lost planet which might afford the mri one more chance.