The emphasis is on sisterly incest in Joe Sarno's VIBRATIONS, cranked out in NYC during a very busy period in 1968 when he was shooting movies all over the place, in Florida, Sweden and upstate New York.
Actors are not credited, and it's mainly a no-name cast plus Mrs. Sarno as a grumpy cleaning lady/comic relief character. Film was made probably back-to-back with the recently unearthed ALL THE SINS OF SODOM, featuring an overlapping cast and same location.
Corny gimmick (a porn staple) is the sisters Julie and Barbara listening to sex through the building's thin walls. Masturbation and soft-X simulated sex scenes do not feature full frontal nudity, but are arousing nonetheless. Shooting in 1968 is established by a Times Sq. theatre marquee advertising GRAND SLAM, released in NYC early that year.
Femme cast (Mrs. Sarno is in a non-sex role) boasts big tits, the calling card of this particular film, and they are spotlighted in high-contrasted b&w visuals. There is less plot than in the usual '60s Sarno soap opera, indicating this was a quickie. In fact, a lesbian orgy over at the neighbors' apartment is shot as pure porn.
The lady using Sarno's patented oversize vibrator resembles a Diane Keaton with big breasts. The prudish & mousy one of the two sisters (Barbara) eventually breaks down and joins in a climactic orgy.
Surprise is a scene of bondage and sexual torture (too much vibrator action) at the orgy. Twist has Barbara falling in love with a stud she encounters at the orgy. The main suspense revolves around how the sisters will resolve their incestuous behavior.
Cheaply presaging a device he used in dozens of his lousy '80s XXX videos, Sarno has random footage of sister Julie wandering around Manhattan to inject exterior transitions. The nasty ending consists of Barbara running off with the stud, leaving sis Julie in the lurch. Julie becomes a full-fledged lesbian, addicted to too much orgasms.
As the dialog quoted in IMDb (rather insulting I would contend) implies, this is substandard work. But Sarno has acquired a relatively loyal following thanks to his repetitiousness which coalesces into such a distinctive style -nearly essential for anyone to achieve status as an "auteur".