Shipwreck Island by S.A. Bodeen

Sarah's father marries a flight attendant from Texas, and now she has a stepmom and two stepbrothers. Her dad decides the new family will get to know each other better by taking a trip to Fiji  and sailing in the Pacific Ocean for a few days. However, Sarah misses her deceased mother, and she has no plans to like these strangers trying to join her family. The long plane ride is boring, the little hotel is disgusting, and the sailboat looks like it might sink. The crew of the boat consists of a gray-haired skipper and his huge pet dog. When the wind and rain start blowing, Sarah knows they're in big trouble. But the real danger awaits them on Shipwreck Island.

The conflict is familiar with a young girl battling her feelings for a dead parent and her reluctance to accept a stepfamily. Being shipwrecked on a small island isn't an unusual plot. However, the strange things found on the island are very unexpected. I always told my students that authors set ground rules for what will be allowed in their books (magic, etc.), but this book doesn't do that. The plot is realistic fiction for the first three-fourths of the book, but then the author starts throwing in unreal creatures. That kind of bugs me. Be prepared to read the second book if you read this one.