This is a middle grade book probably published before 2008. It's about a boy who likes science and has an old dog and a teenage sister. The family goes to visit their grandfather, who's dying, and while there the boy discovers a light or something in the attic/grandpa's office. The boy then uses the light to de-age his dog back into a puppy and his angsty sister back into a pre-teen. He then tries to convince the grandfather to de-age himself to be healthy again, but the grandfather tells him not to. I remember the scene where the sister got de-aged because she was really different and kind of confused, since she had drifted away from the friends she had at that age and was suddenly signing up for high school classes. She called up some old friends and kind of made up with them, I think. I also think the boy had a microscope and a fort in his bedroom that he hid the light in while trying to figure out how it worked. I can't remember the title of this book, who wrote it, or what it looks like. Please help!