Perri's Gates

Sixteen-year-old Perri, on the world of Harmony, is unable to control her Talents. She and her twin, Peter, meet an alien, Cray, who has come to their world through a Gate.
When Peter is snatched by another alien, Perri must go through the sea Gate to find him and bring him back.
Rejoined by Cray, the three work their way though several other worlds to return to Harmony, only to have to deal with more aliens.