The deaths of the parents
Sometimes death can be an inciting incident, but this plot seems so cruel it's ironic. The death of the father would have been difficult enough, but after the death of the mother, the children must certainly wonder why this had to happen, what it might have meant.
The cement burial
Instead of burying their mother in an honorable way, the frightened children, not wanting to go to foster care, encase their own mother's dead body in a case of cement in the cellar.
The question of family roles
Perhaps not surprisingly, the children try to fill in the roles of their parents so that the home still functions, but very surprisingly, this means that Jack suddenly finds himself playing marriage with with sister, Julie, who is starting to seem very attractive to Jack. This question of incest is another layer of irony in the family's story.
The irony of the stench
Perhaps this is a symbol for the lack of solace the children experience, having poorly dealt with their parents' deaths emotionally. Basically, the body of their mother decomposes, and it seems concrete, the way the children did it, will not work to contain the stench of their dead mother. There are many ironic elements in this, the main one being that instead of mourning their mother, the children are forced to remember their mother's death by her literal rotting body.
The ending
The already macabre story becomes even more confusing and vile when in the end of the story, the siblings end up having sex, getting caught, and then continuing to have sex while someone finds the dead mother and calls the authorities. Hopefully, it doesn't need to be said just how ironic this ending is, especially given the unabridged incest.
