Faerie Tale

Faerie Tale is a novel by Raymond E. Feist, Published in 1988.

Plot intro: (from wikipedia)

Phil Hastings and his family have just moved back to his hometown in upstate NY for some much needed peace and quiet from the Hollywood scene. As Phil's twins, Sean and Patrick, soon discover, there is more to their new home than was expected. Gloria, their mother, senses something, but simply dismisses her concern as stress from their recent move. Gabbie, their older half-sister, meets the man of her dreams, but also is tempted by other men. Deep in the woods, The Bad Thing and his Master are ready to break free of the centuries old compact made to keep the Faerie world and the Human world at peace. Only through believing the insane and impossible can they save both worlds from colliding again.