Misfitz Mystery: Two Tigers on a String by Lacey, Josh

This is the second thrilling adventure in the "Misfitz" series, about four children who share four parents and find themselves caught up in some very unusual mysteries. One afternoon, Frank comes home from school to an empty flat. His mother has disappeared. She isn't back that night. Or the next. Where has she gone? How will Frank find her? It's a job for the Misfitz. Together, they sneak out of their home, catch a train across the country and search for clues. They soon find themselves on the trail of a mysterious cult, led by a frightening figure known only as the Tiger.