Metatheater can be a little confusing...
Some more reviewing might be in order, too. So let's see: Esther and Bert's parents first showed up in the comic on page 170 more than 18 years ago now, and popped into the stories fairly regularly after that. Sean's uncle, then, first appears on page 569, just after Sean's initial appearance, in fact, on page 564. And now we all know!