National Lampoons Christmas Vacation expanded the Vacation franchises ensemble by introducing audiences to the larger Griswold family tree. John Hughes, who wrote the first Vacation film and co-wrote the second one (and also wrote the original Lampoon story that the franchise is based on), returned to pen the script for Christmas Vacation. This holiday-themed threequel imagines a disastrous Griswold family Christmas with a burnt tree, an overcooked turkey, and pesky lights that wont go on when Clark wants them to.

https://screenrant.com/christmas-vacation-griswold-family-tree-explained/