Secret of the Pelemnian Stones - N J Schmitt

Eleven-year-old twins David and Derek Tulley are less than thrilled when they find themselves at their elderly babysitter's house for the evening. As they settle in for what they think will be the most boring night ever, the twins have no idea everything is about to change when the babysitter's wheelchair-bound sister, Anna, arrives and offers to tell them a story.Anna uses a special story stone to help her spin a tale that without warning transports her and the boys to the imaginary land of Pelemnia..