Storm Over Bitterhill — Paulette Warren

When Wendy Manson was summoned to her ancestral home, Bitterhill Island, she went reluctantly... and her reluctance turned to terror soon after she arrived!

For Bitterhill was in the midst of Pelico Swamp, a gloomy morass haunted by the memory of Duncan Bitterhill, the slave trader who had built his mansion there. Haunted, too, by many other centuries-old hostilities...

But most terrifying of all was the dreadful curse that seemed to hover over the Bitterhills themselves - Tark Bitterhill, who might or might not still be alive somewhere; his sister, Zay, who sometimes entered another world; and Greg, the black sheep, who might be Wendy's only friend in the swamp... or her most fiendish enemy.

It took the raging fury of a hurricane to bring all the secrets out at last into the light of sanity.