Trying to find a title of a horror/thriller novel?

I recall reading this about 15 years ago, borrowing it from the local library. I believe the title had the word "sister" in it. What it was about was a woman, who's identical twin sister died under mysterious circumstances, I think she fell over the balcony of a hotel. She later finds out that her sister was seeing a psychologist or psychiatrist who uses hypnotism to discover that she was abused as a child by their parents. The surviving sister is puzzled by this as she has a very different memory of their childhood and doesn't believe her parents capable. What she later discovers is that the psychiatrist has planted these memories in her, and has some how transferred is mind to hers, using her body of indecent acts when she is asleep. She later discovers that the psychiatrist has a deformed twin brother who never grew, is deaf, dumb and blind and that he is the mastermind behind this, transferring his mind into his brothers. I guess you would call this novel a supernatural horror. I can not for the life of me recall the title and I have been searching for it. Can someone help me?