I read this years ago, and remember enjoying it as a wonderful gothic romance -- lots of suspense.
Description from back cover:
From her first glimpse of Glen Chandler, young Dina Blake was enchanted. Their courtship was swift, passionate, and filled with excitement Dina never knew existed.
She was certain their marriage would last forever, until he brought her to the family estate at High Towers. Then, slowly, relentlessly, Dina became aware that Glen's twin sister possessed her brother in a way Dina never could.
Suddenly Dina found herself alone in a house of strangers. She could no longer trust Glen's love and she knew his sister would stop at nothing to destroy their marriage. Not even murder.
A superb novel of love and terror, of a courageous young wife's confrontation with ultimate evil.