That's generational wealth. He probably doesn't have to do anything but plays at being a businessman or something. People doing mortgage calculations don't understand how the wealthy live.

The dad’s brother paid for the Paris trip. He had been watching his niece (the older one that counts everyone) while he worked in Paris, and in exchange the brother paid for them to go to Paris for Christmas (and also no doubt so he could see his kids). The brother is loaded, as we also see in the second move, he is remodeling a New York brownstone in midtown.

That says, they do pretty well. It may even be the mom with money. There is evidence to suggest she is a fashion designer with all the mannequins in the house. But the following year when they pay their own way, they go to Florida and not Europe, so they are probably not as loaded as his brother.

Certain activities pay really well.

The NYT did a piece on this.

I honestly chalk this up to Hollywood being out of touch and just assuming "Everyone just has an unlimited food budget that comes from that office everyone works at." as per usual

The official novelisation of the movie says he is a "prominent business man" and the mom is a fashion designer.

So both probably makes quite a bit.

The house costs around $2.5m in today's money, or around $1.1m in 1990. Household income would have to be north of $300k/yr in 1990 dollars or over $600k in today money.

Kevin's uncle paid for the vacation for 15 or 17 people, which would have cost over $40k in 1990. The uncle also had a penthouse in Paris(where he transferred to, creating the desire to bring the family together for xmas.), Kevin's family stayed in a suite.

Given the neighborhood and proximity to Chicago, the father likely worked in finance.

Lobbyist?

It was the 1980s. Capitalism hadn't reached its end-stage yet.

His dad is Richie Rich