I only had read this book from the series, in fact until I looked it up I didn't know there was a series. This book is set that the 2 main characters don't know they are aliens or even that they are siblings at the beginning of this book. Then they find each other, but both had always felt they were different from other people. They can use "mind-speech" to talk. In the end they fiddle with an amulet that sends out a call, and at the end a spaceship appears. The book ends with the 2 aliens assuming they have been rescued, but it doesn't actually tell of the spaceship's inhabitants or its mission.
I don't know if the whole series would be as good, or even if it continued. But I think that for young teens and preteens it is a good sci-fi book for light reading. It doesn't dwell on details or explanations, instead it is fixed upon the characters' feelings on their discoveries and events that happen. So it could be a good book to begin an interest in science-fiction/star-travel books and the questions they deal with: like advanced technology versus a primitive society.