A Shard Of Memory (The World Of Tarra #1) by Aven Tourtillott

Shaellyn came to the bustling port city of Raqut at ten years old, a parentless refugee leading a band of other children who'd survived the same attack. Now twenty, she's built a new life there for herself and her brother Emrys. Then a traveling friend arrives with dire news: the dark mages who took their original home have set their sights on Raqut.
But this time, she's done running.
Shaellyn and her friends have a rare immunity to some of the enemy's magic. This makes them instant targets under the new regime. It also means they might stand a chance at taking the mages down and saving their second home. If they can pull this off without dying, it's going to be a miracle.