YANBU to keep your twins (obviously)
YANBU to be annoyed that your family, having said they'd support you regardless of what you decided to do, are being critical that you've decided to continue the pregnancy.
I'd focus on all the stuff you can do in advance as people have suggested. If you don't already have a big freezer, get one in the autumn so that you can stock up on meals in advance. Freecycle is great if you don't have the cash. It's also a good place for cots if you've cleared out what you used for your older ones. You can put the word out to friends and I'm sure you'll get lots of hand me downs and keep your eyes peeled for NCT sales etc. If you have to double buy it's even more important to get value for money!
As you've said you have a good relationship with your DS's father, try to sort out some arrangement that will give you more support with childcare over the first couple of months. And I know you've said the father of the twins isn't interested, but he is equally responsible for them, so making sure he pays towards them from the start will help cover the costs eg of paying someone to get your younger two to and from school for a month or two. Now is the time to get in credit on the playdate/favour front, as you'll need to cash in around your due date.
Check out any support you might be able to get from sources outside your friends and family, either from surestart or other places like local churches. Not my bag at all, but there are 'pro-life' groups that say they provide practical help and support to mothers who consider abortion in circumstances like yours.
Hopefully, as your family see that you are commited to the pregnancy and being very practical and planning towards it, you will start to get the emotional support you need.