Actually there are four to the National Lampoon Vacation Series; National Lampoon's Vacation, National Lampoon's European Vacation, National Lampoon's Christmas Vacation and National Lampoon's Vegan Vacation. Holiday Road is in 3 of the 4 (it's not in vacation) while Dancin' Across The USA is only in Vacation. Also, they always have had to change the brother and sister because I don't think that they could use the same over and over without them looking much older than they were supposed to be.
Matt