Bringing Stella Home. The Hameji Cycle, #1

In a galaxy ravaged by war, a young man must decide how far he'll go-and what he's willing to become-to save his sister. James never imagined that when his older brother and sister departed on planetside leave, it would be the last he'd ever see them. But as soon as they're gone, a ruthless Hameji battle fleet invades their peaceful star system, transforming it into a war zone. Fleeing with his father on board the family starship, James can only watch in horror as the verdant planet below is reduced to molten slag.
On the way home, James learns the devastating truth: his sister is alive but enslaved. To rescue her, he must make an impossible choice. He's no warrior-has never even held a gun, much less fired one. But to save his sister, he'll become whatever he needs to be-even if it means crossing a line he can never uncross.