This appalling need to gorge themselves on the same pile-high helpings of ineptly prepared and presented food would be bad enough in and of itself were it not for one thing: the dinner table is yet another playing field in the sick, fatuous mental game John and Elly have been playing ever since she found one hundred and ninety pounds of entitled cretin sleeping in her favorite place to moan under her breath about how unfaaaaaaaaaaair and eeeeeeeeeevil her teachers were being by expecting her to follow the same rules as everyone else. This means that every attempt that Elly makes to alter what it is that they gracelessly devour is regarded as a sign of the end times. The horrified look on John's face when confronted with something that isn't the same damned thing that his mother plopped in front of him is a clear indication that he sincerely believes that a change in diet will lead to the end of life on Earth as we know it.
Contrast that with Elly's need to make sure that everyone eats the same thing so that she doesn't have to tax her meager mental capital by cooking different meals for different people and you've found out why she imposes her diet of the week on everyone. It doesn't matter that growing children can no more exist on a slimming diet than a man twice her size, she's not running a diner so everyone eats the same thing.
The interesting and inevitable thing about this specific form of interdomestic polecattery is how quickly it collapses and things resume their usual pattern. What makes it interesting is its appeal despite its inability to work. This is because Elly derives certain benefits from the implosion of her attempt to impose her will; not only does she get to hate people for cheating, she gets to nail herself to her cross.