Reading the Wind — Brenda Cooper

The colony planet of Fremont was supposed to be free of all genetically altered beings--a new home for a pure race. So when Chelo and her brother Joseph, along with two other genetically altered teenagers, were abandoned on Fremont, they were not welcome. They vowed to get off the planet by any means necessary. Joseph and the others managed to escape, but Chelo was left behind with her new found love, forced to live underground.
Joseph and the others find that their homeworld is full of vengeance. Believing that the people of Fremont killed the teenaged castaways, they sent a technologically advanced mercenary team to Fremont to eliminate the entire planet's population. With the help of Joseph's father, the youngsters head back to Fremont to try to save Chelo.