I guess than they should divvy up the bills according to what everyone uses, including when the OPs stepsons come over, and they can pay according to that.

The OP and her dh have a system where they each pay half of the bills, if their system works for them then the amount of people in the household really shouldn't matter to the DISers
My point was that, the OP contributes financialy to her family (yes, including her stepdses), and if her and her dh have their system that works for them, and they each have their own money to do what they choose to after all the bills are paid, then its not our business, or our place to tell her that its "their" money. If she chooses to use that money to purchase her kids a car, so what. If teh dh chooses to use his money on a car for his kids, or a hunting trip, or a new computer, so what, its the way they have their household set and it works for them, who are we to nit pick about their finances and the way we think it should be run?