The Would-Be Witch by Ruth Chew, 2nd Scholastic paperback printing, TX3343, 1976, 112 pages. Illustrated by the author.
DESCRIPTION:“Suddenly Andy couldn’t see Robin anymore. ‘Rob,’ he called. ‘Where are you?’
A little person- no bigger than a bird- stood beside the dustpan. Andy got down on his hands and knees to get a better look.
It was his sister, Robin! But how did she get so small? (from the back cover)