It would be hard to argue that the Samsa family is not dysfunctional. As has been pointed out, Gregor, the son, is not only financially supporting his parents and sister, but paying off his father's debt. Although there is a hospital right across the street from their home, none of his family make any attempt to cure him. When Gregor's mother asks Grete to bring the doctor (who is never brought), "they communicated by way of Gregor's room" (p. 2309), suggesting the patterns of dysfunctional families, in which no one is able to communicate directly. Instead of trying to help him in any way, Gregor's family do everything in their power to prevent him from leaving his room; his father behaves threateningly, often, and violently, more than once--especially when he throws the apples at Gregor, one of which might be responsible for his death. When Gregor's sister Grete assumes responsibility for feeding him, she feeds him garbage: "old, half-rotten vegetables; bones left over from the evening meal, caked with congealed white sauce; ... a piece of cheese, which two days before Gregor had declared inedible ..." (pp. 2316-17, emphasis mine). Gregor believes that his sister is bringing him "a wide assortment of things" "to find out his likes and dislikes"; and when "she left hurriedly" he thinks she does so "out of a sense of delicacy"; and when "she even turn[s] the key," he thinks she's doing so "just so that Gregor should know that he might make himself as comfortable as he wanted." Gregor's interpretation of to his sister's treating him with such blatant disgust, as if he were an unwelcome vermin, suggest that his family is already in the habit of neglecting him, and he is in the habit of rationalizing and justifying this--all very strongly suggestive of a dysfunctional family.
Kafka's approach to the story reflects the very ordinariness of the situation, the dysfunctional family, that his story deals with. The science-fiction writer H. G. Wells once advocated that the writer "limit himself to only one marvel ... when everything is possible, nothing is interesting." Kafka limits himself to only one marvel, Gregor's transformation into a beetle, for a different reason than Wells: if there were any more fantastic occurrences, the story would probably seem fantastic and bizarre, perhaps a profound and horrible vision (such as Dante's Inferno); but by limiting himself to one marvel, Kafka makes the metamorphosis seem almost normal, and the Samsa family, especially Gregor, almost take it for granted. When the story begins, the transformation has already happened; in the story, it is more of a pre-existing condition than an event, mirroring the fact that the dysfunctional family, and the pain and shame that it causes, are normal, pre-existing conditions: "When Gregor Samsa woke up one morning ... he found himself changed ... into a monstrous vermin" (p.2301, emphasis mine). (And the phrase "one morning" suggests the ordinariness, the routine, the humdrum-ness, of Gregor's life, into which this has happened.) Situations with dysfunctional families are dull and ordinary, and Gregor's situation as a beetle, and his family's situation with him being a beetle, become dull and normal.
The narrative tone is very matter-of-fact, unemotional, making no judgements, finding nothing unusual, as if writing about the most normal occurrences. It is not by accident that he uses the word "when," or similar phrases, many times: they give a feeling of "this happened, and then this happened, and then this happened," a feeling of humdrum reality: "When Gregor Samsa woke up one morning ... he found himself changed ... into a monstrous vermin" (p.2301). "Just as he was thinking all this over at top speed ..." he hears his mother knocking. The word "when" lubricates the slow flowing of life in this story (which is like the slow flowing of ordinary life), in order to mimic the feel of ordinary life, resulting in a grim, dead-pan parody of it. The word "when," and similar phrases (e.g., "just as ...") also serve to convey Gregor's passivity; for the most part, he responds "when" other people say or do things, or when he wakes up to find himself "changed into a monstrous vermin," rather than acting on his own initiative, letting himself be motivated by anything that he really wants or needs.
This is the effect of the particular way in, and degree to, which Kafka distorts reality in "The Metamorphosis."