Hi, I'm older than you and have lived that life you're living now, and can tell you from my own experience (complete with older stepchildren) that this stinks of you absolutely being used and abused.
You have become a part of his life plans, not entered a partnership with him.
It was normal in my day, girls like me held no value and had no real rights. People saw us as usable, expendable, and why shouldn't a man take advantage of us, if we knew no better. Things have changed, it doesn't have to be like this for you, or your children.
Never mind how you got here, mind how you get a better deal, or better still out, whichever you really want for your future, but it sounds very like it should be out.
This guy is keeping you on a very tight financial leash for a reason. You are useful but not in a partnership. You have few enforceable legal rights. You're there to serve a purpose for him.
Sorry that sounds horrible, but honestly your posts drip with what your place in all this is, and it is not a good place. Your current and future life is so vulnerable and you need to future proof your rights as a parent.
Ignore random fools who judge you, it's an anonymous forum, and you need help.
Ignore those who condescendingly ask why did you do xyz, or how could you be in this situation. It doesn't matter, it's done. What can you do about it and the position you're in is what matters now.
I suspect your marriage is not registered here because it's a religious one that isn't recognized here. If so you're in bigger problems, and ideally need to contact a women's organization that represents your culture/religion. Google is your friend, but starting with your midwife knowing more would be a very good call.
It never crossed my (very young) mind that the mothers of my step children might also have been abused and that's how he had their children in 'his care.' But that's what was actually going on. Could it be with yours?
Getting and keeping me pregnant ensured he had free full time care for them, a young housekeeper, step-mother, sex on tap, and and didn't have to pay support to their mothers.
My labor was always a cheaper option than repair or replacement of appliances. Am I projecting or is this what may be happening to you?
You are being financially controlled, and without child benefit you have no NI contributions to your name. This is going to bite you later, especially when you're older.
I'm so sorry, but please listen to the people who can see the mess this relationship is and think very hard about it, because it's very unlikely this man will want you further down the line if you start demanding financial equality, and he's already 'taken possession' of two of his children, and replaced their carer easily enough, hasn't he? You could easily be next.