Rick Acker. The Lost Treasure of Fernando Montoya (2003) 188 pp. (The Davis Detective Mysteries 2)
While on vacation in San Francisco, sibling sleuths Arthur and Kirstin Davis are hired by Michael Franklin, a friend of their Uncle Connie, to investigate a lost treasure, putting at risk Franklins family business and, perhaps, Kirstins life. Juvenile.
Setting: San Francisco