So what are you saying? He doesn't have to pay for his first two?
@LaceyBetty
I don't think anyone supports him not paying towards his first two children BUT we don't believe it's a stepmum's responsibility to step in and fill that void, certainly not over and above their actual mother who does not work.
Unfortunately when parents split up they lose the ability to have much influence over each other's lives and choices. If they were still together and had had these twins, of course they would have made an agreement as to how they'd be cared for a who would work to pay the bills. In this case, the dad has made that agreement with his wife, the mother of his twins, about what is best for their household. Right or wrong though that decision may be, OP can't influence that choice.
However, to have 1 year old twins with someone else, ex and OP must have been split up 2 years at the very least. I'd argue that during that time OP could have forseen that his circumstances may change, resulting in less income for herself from him, and that she could have sourced at least part-time work for herself.
My point is that when you're no longer together you can't always rely on your ex to maintain the status quo and certainly not to keep you afloat. You need to have some level of independence which would allow you to support yourself and your children if his circumstances were to change.
As others have pointed out, OP's younger child has been entitled to 30 free hours of nursery for at least a year and a half....that's 30 hours OP could have been working a PT job, plus the weekends when the children are with their dad. Then she would be in a far better position now.