Calum Iain MacIver’s review published on Letterboxd:
Known in English as either "Sexy Sisters" or "The Devilish Sisters", "Die Teuflischen Schwestern" is a strangely interesting effort from Euro-sleaze maestro Jess Franco, who appears to have virtually dispensed with plot in its entirety in order to focus on artfully staged surreal softcore frolics. The plot revolves around heiress Millie von Stein (Karine Gambier) who is being "looked after" by her older sister Edna (Pamela Stanford). Edna is also the executor of the family fortune that Millie will inherit when she turns 21 – unless she is declared insane. Edna and her lover Dr. Barnes (Jack Taylor) covet the inheritance and devise a plan to drive Millie mad by injecting her with drugs that turn her into a raving nymphomaniac. They shackle Millie to her bed and have sex in front of her in an effort to drive her mad with sexual frustration. Edna also arranges for men to have sex with Millie and afterwards convinces her that she hallucinated the sexual encounters. Things begin to go wrong for Edna, however, when she picks up a guy in a bar and invites him back to the mansion. Unfortunately the bloke immediately falls madly in love with Millie and begins to contrive a plan to rescue her before she goes completely over the edge. "Die Teuflischen Schwestern" is a totally ludicrous but fun little film. Jess Franco's direction is reasonably straight forward, avoiding most of the shaky camera, odd focus and violent crash zooms that were to become the "trademark" of his latter career. There are plenty of Franco signifiers in the film, however, particularly the opening sequences where two strangely masked naked women perform some sort of avant-garde show in a smoky nightclub. Some individual scenes are nicely shot, with a detached artfulness by cinematographer Peter Baumgartner, particularly the sex scenes, all of which help give the film an odd, dreamy air. The design of the film, particularly the von Stein mansion is also impressive – all feline pictures, strange beaded curtains and Millie's literal gilded cage. All the performances are fine, although the actors have very little of substance to. Karine Gambier deserves special mention, however, for delivering a memorable performance; the vast majority of which is spent in the nude writhing manically in her bed. Walter Baumgartner provides a quiet, subtle and subdued score which works well with the overall approach. “Die Teuflischen Schwestern" isn't the strongest of Jess Franco's many (many) films but it is an artfully enjoyable effort, with a dreamy languorous style that foregrounds nudity and a totally mad (if very slight) plot line.