Half Magic

This book is about four children and the second yungist finds a nickl and when someone has the nickle and they wish for someone their wishes come half true. They wish for all different wishes, like they wished that they were at the time of Camolot true times more. And wanted to go to the desrt and when they came back from Camilot, they went to the movies an the oldest sister was mean to the youngest sister so she went under the oldest sisters chair. Then she wished she wasnt even alive so she got dimer and dimer till she looked like a ghost. Then she went threw the mall and everyone thought she was a ghost so she turnd into the ghost. I'm going to have to tell you that 7 is a magic number so thats how many days she can get back to her normal self but her trip she already had was a cupel days. Will they prevent her from disappearing?