multiple universe theory
may explain why the kids are different in each film
shareI've heard some say that each Vacation is meant to be stand-alone and not connected to the others... BUT I think the different kids is just a running gag.
It was so apparent in Christmas Vacation that Rusty was the younger brother, whereas in the first 2 I thought he was older....I used to think that MAYBE Clark and Ellen had another son and maybe in Christmas Vacation older Rusty had moved off to college or something and the Rusty we see in that one is the another YOUNGER son....yeah I doubt he'd be named Rusty, but it's just want I used to think lol
The kids kept aging so they had to recast them, as for why Audrey is older than Rusty in Christmas Vacation its a continuity error that they were hoping the audience would be too stupid not to pick up on.
shareRusty was younger than Audrey in all of the movies; just look at the ages of the people who played those roles. The Audrey actress was always about two to five years older than the Rusty actor, even in the 2015 movie.
The average male surpasses the average adult female height when he's in his early to mid teens, which is why Rusty was taller than Audrey in the first two movies (and for some reason, people assume taller = older, even though that line of thinking has no basis in reality). In the third movie they cast a runt to play Rusty (Johnny Galecki), who never exceeded the average adult female height even when he reached adulthood himself. Juliette Lewis is still taller than he is.