L.J. Smith has written a few teen series about the paranormal. This book is one of her earliest works, part one of a duo of books about 4 siblings. It is geared more towards a younger pre-teen audience. The Hodges-Bradley childen are recruited by a sorceress's familiar to rescue it's mistress. The house of the sorceress Morgana is a gateway to another world and she has been trapped on the other side of the mirrors. If you liked her vampire series and her teen books I'm not sure you will also like this book, it really is for a younger set and can feel cliched (one dimensional bad guy) or cute (talking animals) to older audiences. The 4 siblings panic and fight and get separated a lot which exasperated me.
"The beautiful red fox lures little Claudia to the forbidden house on
the hill. Claudia summons the twins--Charles, the dreamer, and Janie, the genius--and sensible Alys, who never imagines what evil lays waiting for them. Together they find the spell to make an amulet
that lets them pass through the moonlight mirrors."