Question: What did Kevin's parents do for a living to afford that house and a trip to Paris for nine people?
Answer: Adding to the other answer, it was a trip to Paris for more than 9 people; it was Peter's brother Rob who paid for a total of 15 plane tickets to Paris. Two tickets were purchased for 2 of Rob's kids: Heather and Rod (3 other siblings are in Paris), as they were still in school in the States. Six tickets for Uncle Frank, Aunt Leslie, and their 4 kids. Seven tickets for Peter, Kate, and their 5 kids: Buzz, Megan, Linnie, Jeff, and Kevin (his unused ticket was accidentally tossed in the trash during the pizza dinner mess).
Answer: It's never revealed in the film, but director Chris Columbus recently did an interview in which he said that he and John Hughes had discussions about it and imagined Kate is a "very successful fashion designer," and Peter worked in advertising (though Columbus said he couldn't remember if they had a specific job in mind for him).