Adventure in Istanbul — Cora Taylor

Thirteen-year-old twins, one of whom has discovered a magical spell that makes her temporarily invisible, wind up on a Greek cruiseship with their grandmother, battling spies and trying to rescue their father. Jennifer Arnold is always getting her identical twin, Maggie, in trouble. Life's been difficult in their household since their pilot father went missing, and things get even worse when Jennifer finds an ancient spellbook and learns to become invisible. Then the girls get a wonderful offer - their Grandmother Arnold takes them on a cruise to the Greek islands and Istanbul. But Grandmother Arnold seems to know a lot more about the world of espionage than an ordinary grandmother ought to. The kids learn that not all the people on the ship are who they seem to be either. What's going on? The holiday cruise quickly turns into a mission to find the girls' father. With each stop, the kids learn new information about his disappearance and the danger he's in.