Jungle Jeopardy, by Mary M. Sikes
When Dana realizes Clifton is missing, she takes off for Costa Rica to find him. An apparent kidnapping turns into a jungle adventure that leads Dana and Clifton into the wilds of Guatemala where they discover an unexplored Maya cave, find pottery and walls covered with glyphs.
Tyler Hunter wants to save Dana’s sister from prison, and to do so, he must find her grandfather’s hidden treasure. An archaeologist on sabbatical, Tyler intrigues Dana with his knowledge of the Maya.
Dana has never doubted her sister’s guilt in the murder of their grandfather on the island of Antigua, but now an element of doubt begins to creep in.