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What evidence do you see in the movie that there isn't four incomes coming into the household? What evidence, on screen, is there for his dad being the only income?
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In the first one it's Peter McCallister's brother that flies them out to Paris as a gift. A third brother that is never seen but talked about. >

His brother is probably in the mob. Their house was nice but not INSANELY expensive. Zillow estimates it’s value at just under $2.2M right now. For a 6 bed/6 bath with nearly 6000 square feet only 30 minutes north of downtown Chicago, it’s not cheap by any means but it’s not wildly ostentatious either, it’s just a little ostentatious >

Lol yea so back then it was probably worth 100k >

It was established in the first movie that they're an upper class family.
Harry said they got onto that block in the first place so they could hit that house, meaning he knew they were very well off. >

A poor family can't afford a house that big. >

Their house is in winnetka, IL (IRL). No sketchiness, just a good job. >

This has been around and I think I read he is in sales. 
Which is believable. My sister lives in a very affluent neighborhood. And most of the husband's who make bank are in roof sales. Who'd a thought >

Huh, and all this time I thought buying a new roof was super cheap. >

Well… let’s do the math… 
Today the house has an estimated market value from the Cook County Assessors of $1.5 million. The median price of a home in the suburban Chicago town of Winnetka is $992,500. The median income in the neighborhood is $218,542. 
If you bought that house for $1.5 Million and put $300K down, then your payment would be about $6,300 per month. Don’t forget another $1600 per month for taxes and insurance.
The house is 4,243 square feet and built in 1920. It’s really big and really old, close to 100 years old, so they’ll be plenty of upkeep. For maintenance, upkeep and cost of utilities, you’ll need to tack on another $3000 per month.
We can probably assume they have a gardener and some sort of handyman/hired hand to help out. 
So we could probably estimate on $11K per month just to own and properly maintain the McAllister’s House.
Based on that, let’s say his mortgage and total cost of keeping the home was only 1/3 his take-home pay.
That would mean he is bringing in $33K per month after taxes or $396,000 per year. That’s $575,000 pretax, in today’s money. 
Today a CEO in Winnetka, IL could make from $625K to $1 million, according to Salary.com.
Todd Strasser, wrote the novel based on the movie, describes Peter as a prominent businessman and Kate (Kevin’s mom) is a fashion designer. Today a fashion desginer in Winnetka could make around $100K per year.
Prominent business man could easily be CEO of a company. >

He is a government employee getting paid under the table as a mob informer. >

He's Dell Griffith's cousin, and is the leading shower curtain ring salesman in North America.