It so happens that this is first cousin to another irritating affectation whose purpose seems to be mostly about shaming her family. The crime that she's trying to combat is their having the idea food can be liked or disliked. Not only are they wasteful, their rejecting food on the irrelevant quanta of flavor and digestibility is another means by which they turn their backs on the love in her great big heart. In order to defeat their evil hope to make her mother show up and lecture her about being a bad child who wastes food, she obsessively wolfs down the unpalatable crap her children simply refuse to eat.
The reason for this, of course, is that she's more like her mother than she cares to admit. While it always alarms and sickens her when she looks in the mirror and sees a face like her mother's staring back at her or catches herself saying or doing things to her children that she hated when Marian said them or did them to her, the plain fact is that the reason she and her mother didn't get along at all well is that they're too damned alike to coexist for long. Since she's stupider than her mother, I should think that her concern is at best misguided.
This is because while I can picture Phil and his mother getting into a tiff because he actually had a sense of taste and wouldn't gobble down horrible English cooking that is horrible because England's gift to the culinary world is stodgy garbage that causes dyspepsia, it seems to me that Marian really didn't have to lecture Elly too much about not eating everything on her plate because Elly's guilty conscience couldn't bear to deal with left-overs. Hmmm. It just occurred to me that maybe Marian actually lectured Elly about eating all the stuff Phil wouldn't eat because that's kind of nuts and also, how was he supposed to learn to not be wasteful if she gobbled down his liver and Brussels sprouts?