JsMa,
Yes, in PA they treat the children equally now -- it says so on the main CS website. So your babies will be supported if you request it. And the previous order will simply be reduced. Again, not your problem. I mean, think -- if a couple were married and the man had another child outside that marriage, he'd still be liable to support that kid. We wouldn't hear any nonsense about how his wife didn't budget for that. She'd get a ton of sympathy, but no one would ask that the new baby not be supported. The kids would all be supported. And, ironically, these Dark Ages "first to the well" rules in some places could backfire (in this example) against the legal spouse (or STBX?) if the other baby mama filed first. But I digress.
Oh, and forget about fairness. State guidelines vary wildly, so they can't all be fair. They're based on someone's approximation when the laws were last updated in each state. Fairness is too high of a bar. For example, DH has to pay, let's just say, a rather LARGE amount of support in our state, which doesn't take the CP's income into account. In your state, according to the online calculator, she would have to pay him (since she makes so much more money and he pays health insurance). Are these both fair? Doesn't matter. They're both the law, and it just matters which place has jurisdiction. Just go with the law where you live.