ret con by omission
Putting aside the actors' real life ages, just going with the timeline of this movie, Egon's first grandchild would have been born no later than 2002. Which would mean his daugher would have been born no later than the mid-1980s.
Yet in both the earlier films, there's no mention whatsoever of Egon having kids and is strongly implied to be a bachelor - there's the hint of romance with Annie Potts' character in the first one and mention of his sleeping with mood slime in the 2nd one. So where's this family suddenly come from?! The only answer is the old "well, we didn't explicitly say that he DIDN'T have a family in the earlier movies". Come on.
I can't help thinking this might have played better if they'd had an older actress playing Egon's estranged wife/widow with Egon's teenage/pre-teen kids. But then you wouldn't get Paul Rudd flirting with an attractive younger actress, because that's what cinema goers want to see
And while I'm at it, I don't remember any references to the events of the 2nd movie at all in this. So ret con by omission in more ways than one! Let's pretend Ghostbusters 2 never happened (at least in the video game, they have Vigo's portrait in the basement and talk about how Shandor was the architect of the museum etc)
As I said in my last post, sorry if I'm covering old ground here. I'm not trawling through 16 pages of threads to check!