People talking about the taxpayer as a factor in this decision can go fuck themselves. Sorry, but if you seriously think someone should abort just for that reason then you disturb me most profoundly, and at least if you fuck yourselves there is no risk of your troubling the gene pool further. (Though admittedly I'm not that good on reptilian reproduction; cold-blooded creatures may produce asexually for all I know. If so, consider my advice withdrawn.)
OP, there's a multiple births and families thread on MN and I think popping over there for experienced wisdom and advice might be a plan. Honestly, I'm a decade older than you and twins would kill me from lack of sleep alone - you have youth on your side! And you've coped with things that would break many people already, and above all, you've made your decision. Obviously this isn't ideal or how you planned life to be, but it's the situation, soo I think the plan now should be how to set up support systems that will make things manageable for you, going forward. If you can get your littlies to stay over at their Dad's a couple of schoolnights a week well before the twins arrive, so they don't connect the two, they'll get more sleep before school and you'll have less frantic mornings. Surestart and Homestart sound great too. And I think TAMBLA might be able to offer assistance in terms of discounts etc. Also, LIFE (the anti-abortion charity) would fall over themselves to offer you free baby clothes and equipment and supportive ears etc - I disagree with their position, being staunchly pro-choice, but they are very strong on helping women in crisis pregnancies which I can respect. They are in effect making it more of a choice for women who lack support.
Is the babies' father in employment? I hope so. If he can't man up enough to be a father, he can at least contribute financially. And I also hope that once they're here and real little people he may change his tune. You can't rely on it, obviously, but stranger things have happened and lots of men play the "don't want to know" card to apply pressure to abort.
Hang on in there. Your kids are at least schoolage, so this could be worse.