Nice try, but you didn't answer my question.  How do both of them pay more than half the household costs?

OK, I'll give you a hint.  Mary is a teen-ager who like her mother is a vegan.  All that designer food is expensive.  Dad refuses to pay for it.  So Mary and Mom do their own shopping and Mom pays for all of their groceries.  Meanwhile, Dad buys all the food for son John.  Lots of meat and dairy products.  They have separate refrigerators.