Although he had made a few films before,Sisters is perhaps the first film in the true De Palma style,you know,the obsession with voyeurism,the show-offy camera-work,the Hitchcock homages,the preferance for long,wordless dreamlike sequences interrupted by bloody violence,etc. Although De Palma HAS made films that are different,he seems to prefer making films that are like this. Sisters has all these elements,albeit sometimes in embryonic form.
The model here is obviously Psycho,and De Palma doesn't develop elements of that film as well as in the later Dressed To Kill,but it is interesting to watch. The director just about manages to have enough different elements,while riffing on obvious things from Psycho {such as a shocking murder a third of the way in,the basic plot} and even a bit of Rear Window. Things like split screen keep the interest despite a few dull bits,and towards the end De Palma is confident enough to resolve most of the film with some almost surreal dream/flashback sequences and an amusing pay off.
Accompanied by a sometimes TERRIFYING Bernard Herrmann score {nowhere near his best,but incredibly effective},and with plenty of humorous touches and an interesting feminist heroine,Sisters is nowhere near top De Palma,but it's a good dry run for classics like Dressed To Kill and Blow Out.