teen fantasy fiction book: please help!!

[fishie]

I read this book around 1997-99 in Australia (but don't believe book was set in aus): the book cover was a wall of wailing, tortured looking faces (scary!) It was about a brother and sister - went to live with Aunt (Bee or Bea maybe??) Creepy forests or woods close by. Brother is lured into forest by charismatic, nasty other -worldly teenage boy. Big sister goes in search of brother. Turns out forest is enchanted (time kind of stands still), full of dark creatures, scary children who live in forest - they look like normal children but from another time, lost in the woods long ago but remain the same age and are now cruel etc. Other strange creatures live in forest. Sister and strange teenage boy share a bit of chemistry - turns out he knew her mother when she was young and before he himself got lost in forest and became a stooge for some evil being. wall of faces comes into it somewhere - anyway help!! been on the hunt for this book for ages!