First of all, we have no idea if they are equal gifts.
Absolutely. Generally, a used pickup truck is more expensive than a used car. In addition, given that the truck can't be legally driven for at least two more years, its value will depreciate more slowly than the used cars the OP bought the children to whom she gave birth. And the truck is less likely to be in an accident, and it costs less to operate. Second, the parents are putting out a large amount of money for one set of kids not the other
No, the OP has paid some unspecified amount of money - likely savings/earnings from before she was married, i.e. her money - to purchase used cars for children whose sole support she had been -- who cares what the aunt arranged? The mom is still making it clear that she will spend money on one set she won't on the other -- in fact, she has said several times that she would give money to the step kids cars if their mom did the same (1/2 of 1/2 -- what a sport).
Half of half? Where are you getting that? Do you mean that IF the aunt doesn't buy the boys the cars she promised, the OP and her husband would pay half if the boys' mother would pay half, for each boy's car - so that, technically, she personally would be paying a quarter? That's ENTIRELY reasonable. She obviously had no such restrictions on her own kids.
That's right, she had no such 'restrictions' on her own kids. She paid 100% of the price of their cars, with no expectations of anybody else paying a single cent toward the cost of those cars Kids know the difference, and they see the unequal treatment. Maybe you can ask the OP why the step kids are not worthy of their own ages in her siggy.
ARGHHHHHHHHHHHHHHH!!!!!!!!!!!! Talk about different treatment from the beginning. Just listen to how she talks about them. As others have said, this goes well beyond cars