Oh, Hogwarts, what an emotional rollercoaster this whole thing is for you!
Please, please, please tell your family about your pregnancy as soon as possible.
It seems to me that the reason you are having such a hard time with this is due to the manipulations of your partner. Until you create a support network of Real Life people who would be supportive of you continuing this pregnancy I think your partner will continue to pressure you because you are isolated, therefore vulnerable and I've no doubt he can clearly see his manipulations are working.
He'll probably tighten up the pressure on you the most just before you break your silence because he'll be terrified of losing control of the situation once you spill.
Hogwarts, you may well genuinely want and benefit from a termination, but how on earth can you be expected to make a balanced, reasoned decision under such duress? This is why you need people who will support you if you do decide to carry on with the pregnancy- you have so much pressure from one direction, it would at least be more balanced to have some from the other direction. Mumsnet is not a substitute for the people who love you supporting you through this difficult time. Tell your mum and dad. Tell your brother your husband is bullying you into a termination when you aren't sure you want one- I bet his protective brother instincts will override any wedding selfishness!
Your husband is presenting you with something called a false dilemma. It is a logical fallacy where you are told you must to choose between one option and another when you actually have other options- your partner is posing it as your kids versus your potential kid. The thing is, it's not a case of your kids versus potential kid. Giving another sibling to your children is a gift, not a curse! Sure there'll be adjustment, financially and emotionally, but I doubt that your kids will grow up and tell you that they wish their little brother or sister was never born and it was the biggest mistake you ever made as a parent, do you?
If your husband decides to leave you then that is his (or rather, his lack of) responsibility. Don't let him try and make you responsible for his decision. Despite his outrageous behaviour towards you during this pregnancy and his unfaithfulness previously you still want him to be with you. He should be on his knees thanking you for being such a forgiving and understanding wife, so don't be bullied by his threats. If he leaves such a wife whilst she is pregnant with his third child, then that is for his conscience, not yours.
Now, please, pick up the phone. Reach out. Good luck.
diz x