Marco's millions

Marco's Millions is a book by William Sleator. It is a prequel to the main book, The Boxes. It is about a boy who likes to travel. He often secretly rides buses far from home, though only his telepathic sister finds out. One day, his sister sees strange lights in the basement, and she and Marco investigate. They find a portal into another dimension, and thus the adventure begins. But when Marco comes back home, 23 years have passed and his sister is dead.