Calling Brian De Palma's films derivative would be like accusing George Carlin of being crass. Of course they're derivative; his modus operandi consists of cataloguing genre conventions and the characteristic techniques of the masters of the genres he works in. Towering highest among the geniuses of suspense is, of course, Alfred Hitchcock, and De Palma established himself as his heir by taking his narrative formulas and visual techniques and developing them. For Sisters (1972), De Palma even hired Hitchcock's long-time composer, Bernard Herrmann, rescuing him from the relative obscurity into which he had sunk since Marnie (1964).
Sisters is essentially Psycho meets Rope meets Vertigo meets Rear Window—and that's not merely an indirect way of saying that it's like a lost Hitchcock masterpiece. It combines distinctive elements of each: Psycho's narrative red herring whereby the protagonist gets killed early on, as well as its obsession with bathrooms and its extreme closeups of eyes; Rope's purloined body; Vertigo's geography, with Staten Island serving as an East Coast San Francisco with its steep hills descending to the ocean; and Rear Window's observer-with-binoculars vicariously participating in a search through the murderer's apartment. De Palma also acknowledged the influence on Sisters of Roman Polanski and Max Ophuls, respectively on its dream sequences and its carefully choreographed tracking shots.
Above all, Sisters is the quintessential Staten Island movie. Saturday Night Fever (1977) may give love to the Verrazano Narrows Bridge, Panic in Needle Park (1971) may portray the fifth borough as an ersatz Eden for the junkie protagonists to dream of escaping to, and Goodfellas (1990) may have been shot there, but only Sisters takes place there almost in its entirety (with short excursions into Manhattan for the Time-Life building and the "famous" African Room club/restaurant on West 44th Street).
The Zeitgeist of 1972 is nicely captured in the character of Grace Collier (Jennifer Salt), who makes a living writing exposes of police and city government corruption for The Stated Island Panorama. "Why don't you go beat up some students," she sneers at a couple detectives. Sisters also proudly continued AIP's tradition of hokey gimmicks; the posters warned, "To allow audiences to regain their composure from the emotional impact of 'Sisters' after each showing, no one will be seated during a SPECIAL SHOCK RECOVERY PERIOD!" Alas, home viewers will have to recover from Sisters on their own.