The 13th Floor — Sid Fleischman

There's never a dull moment in the lives of twelve-year-old, newly orphaned Buddy and his lawyer sister, Liz. Their wild adventures begin when they receive a message on an answering machine from an ancestor named Abigail who has been dead three hundred years. She's being tried for witchcraft! Buddy and Liz find themselves on the thirteenth floor of an old building and are soon transported back in time to the pirate ship of another ancestor, Captain Crackstone. Buddy hopes to find treasure to pay their mounting bills, while Liz hopes to save Abigail from being hanged as a witch. Can Buddy and Liz change the past and find a way to get back to the future?