The reason if, the other two sisters are variations of monstrous people who are successful: Karin (Ingrid Thulin) is what seems to be an icy control freak, not a horrible woman, but close, who cannot stand human touch of any kind, and we're privy to her torment in a flashback where she uses a piece of a broken wine glass to literally mutilate her vagina, seemingly enjoying it (as she shocks her husband), while writhing in unspeakable pain. Maria (Liv Ullmann) shows less, is more of an enigma and a hard character to pinpoint, but she's equally manipulative if not flatly repulsive: she not only tries to resume an affair with Agnes' doctor (who points out she her internal ugliness is starting to show through her facial expressions as she apparently enjoys this criticism) but she also fails to help her husband at a critical moment when he stabs himself in the stomach. Later on, both sisters (particularly Karin) verbally express their hatred for each other in a powerful montage that leads to a moment of equally intense reconciliation that comes without the use of dialogue and is emphasized by the swell of music (Chopin). Ingrid Thulin stands out, letting the gamut of Karin's sheer rage our out of her visage while suddenly changing back to a softer self, then to her steely image of self-control and back to fury in a snap of fingers.
There's a lingering question throughout the movie: there must have been some extreme trauma, some truly horrific event (or events), that must have set forth the deluge of pain that rips throughout the entire story right up until the end. Karin herself alludes to the "tissue of lies" that could mean anything: possible sexual abuse by the unseen father, a complicity between Maria and her mother, and Agnes in the center, rejected. Although Karin does mention the revulsion she felt towards Maria, one can only wonder what it was that transpired which seems to have driven her into momentary lapses of insanity. In fact, her self-hatred mirrors Agnes' intense suffering rather closely, although both characters rarely interact and are on entirely different planes. In some ways, it wouldn't be out of place to say that Agnes, despite the unimaginable suffering she endures, serves as merely a catalyst, a materialization of the horror Karin herself has had to face. At least Agnes has experienced a form of love, if selfless, under the guise of Anna who comes to her aid, gives of her breast in an almost saint-like passion. Karin only has the cold future of her own dismal life to face. And Maria isn't even alive, but sadistically observing.
Aside from PERSONA this has to be one of Ingmar Bergman's most powerful works, one that hits an audience at a gut level (cliched as though it may sound). The dominant color red only adds tension to the already tense scenes between the quartet of women (and the men, who only make pat but indelible appearances). Woody Allen would use these stylizations of the face looking dead at the camera, telling a story of its own in INTERIORS, another looks at three sisters, neither of them sympathetic. Grueling, sometimes unwatchable, this is movie watching at its best.