Trapped — S.A. Bodeen

Sarah Robinson and her family are shipwrecked on a remote and mysterious island. Their food is scarce and there's no sign of rescue. They have seen strange creatures, rescued a mysterious girl, and found The Curator, who has captured Sarah's father and stepbrother to use in a bizarre time-travel experiment. And then the only man who knows about the island comes back―he's looking for buried treasure and won't leave without it, even if it means leaving the Robinsons stranded. Sarah knows an important key to finding the treasure, but will she keep it a secret?