Sisters (1973) by Brian de Palma

A charmant lover of equally charming et naughtie Danielle (Margot Kidder) one of the famous, just recently seperated siamese twin sisters, gets brutally stabbed to death [-he was just about to surprise his new love with a really deliciously arranged birthday cake-] right in front of the eyes of nosey journalist Grace Collier (Jennifer Salt), who, in sherlock manner gets on the case and directly into the hypnotic hands of the sisters’ psychologist Emile Breton (William Finley).
Desperately trying to seperate Danielle from the evil presence of her sister Dominique, Breton himself gets killed, again just right in front of the eyes of Collier, who, to everybody’s surprise, hasn’t seen anything.

seen at: pickledfeet private video collection