Yes, it was the uncle, so it's a money family, money comes from the previous generation. They don't ever mention their jobs in the film, but in the novelization the mom was a fashion designer, but that's something the author made up on account of all the mannequins. He made the dad a businessman because it was a safe bet.
It's at the beginning, when Joe Pesci is casing the house, disguised as a cop. From the script, via screencraft.org:
KATE
(to the cop as she pays
the boy)
I'm sorry. We're going out of town
tomorrow and it's just crazy around
here. My husband's brother
transferred to Paris during the summer
and his daughter goes to college here
and his son is finishing high school
and staying with my brother-in-law.
Kate gives the last of the cash to the Pizza Boy.
PIZZA BOY
Thank you.
Nice tip.
Merry Christmas.
He exits.
KATE
You, too.
Kate continues explaining to the cop.
KATE
so ••• as a Christmas present ••• My
husband's brother gave us all a trip
to Paris ••• So we can all be together
for the holidays. My brother-in-law
drove in from Ohio today ••• we all
leave for Paris in the morning. We
have a house full of people here
and ••• (referring to Kevin)
••• this one's a little out of sorts.
Give me a minute and I'll be right
with you.
It was a throwaway line early in the film. The uncle paid for plane tickets for all the family members to come see him in Paris, and IIRC the adults were flown first class while the kids were flown coach.