S. A Bodeen

Sarah, Marco, their families, friend Leo, and his grandfather continue to strive to outwit treasure-seeker Fox and escape the mysterious island and its strange inhabitants.

Shipwreck Island, Book 1:
Sarah Robinson is deeply troubled in the wake of her dad's second marriage. She now has to deal with a new stepmom and two stepbrothers, Marco, who is her age, and Nacho, who's younger. Even though they've all moved from Texas to California to start life as a new, blended family, none of the kids seem remotely happy about it.
Sarah's dad and stepmom then decide to take the whole family on a special vacation

...

Robbie's last-minute flight to the Midway Atoll proves to be a nightmare when the plane goes down in shark-infested waters, but the real terror begins when the co-pilot Max pulls her onto the raft.

Sarah Robinson and her family are shipwrecked on a remote and mysterious island. Their food is running out, and their fear is escalating as there is no sign of rescue. The mysterious girl they found unconscious at the beach is healing, and what she tells them about the strange island and especially about someone called the Keeper has the family on edge. When Sarah's dad and Marco's younger brother go missing, the mystery becomes dangerous. Now, it's...