Both your cars are relatively new, and while you haven't provided the details, you seem to have figured a savings of 12.5k which is not something easily ignored.
I'd say that it makes a lot of sense to sell the auto that is used rarely and obtain another (with cash) when the planned child arrives and your wife gives up her work van.
I'm not sure why you wouldn't want to save some money by doing this now. I'm assuming that your car would be able to handle both children as well as your wife short term if needed (I'm showing my lack of car model knowledge here
) space-wise. But I'd say if the numbers are in your favor, go for it.