This movie is a variation on Kafka's "Metamorphosis" taken to even more absurd extremes. It also took several cinematic & thematic cues from Dali's "Un Chien Andulou." Both of those works, like "Sitcom", are about rebelling of what is often regarded as proper or right in normal society.
The rat, an unusual, off-beat pet in an otherwise normal house, inspires the abnormal elements of the characters' subconscious, causing them to act in the most bizarre fashion, beginning with the son spontaneously announcing his homosexuality at the dinner table, followed by his immediate seduction by the maid's husband, then the inexplicable suicide attempt by the daughter.
After that, the movie gets wierd.
Through all of the strangeness & familial collapse the father remains steadfast that absolutely nothing is wrong that everything will come out all right in the end. And then ultimately his fate is the strangest of all.
There are movies out there that demands that the audience think about them, that in order to get something of satisfaction they have to put some effort of imagination, of their own personality into it, like a cinematic Rohrschach test (was that spelled right?). This is one of those movies. Highly recommended for fans of the Avant Garde cinema.