The Strange World of Mark and Anna — Michael Davies

Mark and Anna meet the day before their thirteenth birthday when Mark moves to a new school. They discover more and more about each other and realise that they must be twins, adopted by different parents. But they also learn that they have unusual powers that show that they are not human. Eventually they learn of the two hundred year old war that has been fought on Earth by two groups from their home planet who were marooned on Earth by a space accident involving the ship that brought them. They learn that their natural parents have been kidnapped by a group that has gone rogue in its quest for wealth and that they are the only ones who can rescue them. Mark and Anna must draw on all their extraordinary powers and their courage to sort out this terrible mess.