I can't even piece together how this could be true.
A family probably only needs one cargo bike. An brand new _expensive_ cargo bike costs around the same as the yearly cost of a car.
If you think your car costs less, calculate your TCO.
And the other 3 members of the family are walking?
For what a single car costs in taxes, insurance, fuel, and maintenance for a year your could get 5 used bicycles and a nice cargo bike trailer off amazon.