Tale of the Secret Mirror, The (Are You Afraid of the Dark? #5) by Brad Strickland, Barbara Strickland

Kim Chen's furious. Her little brother Doug has found her secret hiding place in the attic. She's determined to get rid of him. So to scare him away, she tells him there's a witch in the mirror who will come out if she calls "Mirror Maggie" three times. And she threatens to do just that if he won't stay out of her space. She beings to chant the magic words. . .

That night Kim hears someone creeping up the spiral staircase to the attic. Then she hears a shriek. It's Doug. Or is it? Soon she's swept into a sinister world where nothing is as it seems. . . a place where she and her family may well be imprisoned forever.

Unless Kim can find a way out. . .