Icelandic folklore did not always depict the Yule Lads as the sons of Grýla. In the earliest tellings, they were described instead as her brothers—a family structure that, while still unsettling, makes far more sense.
A troll woman having 13 unruly brothers is far less strange than the later image: Grýla as a mother living in a cave with a fat, lazy, stupid husband, thirteen fully grown sons who never leave home, and a giant man-eating cat. That version turns her into something not just monstrous, but oddly dysfunctional. Frightening, sure, but also strangely absurd.
In the older traditions, the Yule Lads are nearly as terrifying as Grýla herself: wild, chaotic trolls rather than mischievous pranksters. If Grýla were truly their mother, it raises an uncomfortable contradiction. A being who relentlessly hunts and punishes naughty children would hardly tolerate misbehavior in her own offspring—let alone raise thirteen of them to be exactly that.
As brothers, however, the dynamic fits. What is a powerful, fearsome sister to do with a pack of naughty siblings other than redirect her frustration outward? Their unruliness becomes the very fuel for her role as a hunter of misbehaving children, rather than a failure of motherhood.
As with all folklore, stories change over time. And like gossip, later additions often grow louder, stranger, and more distorted. The earliest tellings, though quieter and less theatrical, tend to carry the greatest trace of truth.