4 ideas
After their father dies of a heart attack while paving over the family garden, his widow sickens and dies. Over one hot summer the concrete cracks and the smell spreads, and Julie's older boyfriend Derek grows suspicious. He finds Jack and Julie in bed together, breaks open the trunk, and calls the police.
Once no adult oversees the household, each child drifts toward their most private impulses rather than toward self-governing maturity. Jack stops washing, Tom regresses to dressing as a girl and sleeping in a cot, and hygiene, schooling and time structure all collapse. The implied mechanism is that civilized behaviour is held up by outside observation, not by internal values, and fails quickly when that observation is withdrawn.
When parents disappear, children fill the empty roles instead of leaving them vacant. Julie becomes the mother, Jack the father, and Tom their infant, so the family structure survives while its occupants are swapped. This re-enactment is what erodes the incest taboo, because role logic ('mother and father') overrides kinship logic ('brother and sister').
The father pours concrete over the garden to avoid tending it, and the children later use the same material to seal away their mother's corpse. In both cases cement stands for trying to cover something messy and alive rather than dealing with it. The cover needs constant maintenance, it cracks and leaks, and it hands control of the secret to whichever outsider notices the rot.