None of us can tell you what the “right” thing to do is. I think you need to sit down and have a proper discussion with your husband about what you want, what he wants and the practical issues you are going to have to face one way or another. I think you could also do with some counselling either way.
Ultimately, you get the final say. I do think you need to consider your husband’s point of view- perhaps if you understand why he is against it, that would help? I guess if you really want this pregnancy, and he remains against it, one if you is going to be unhappy. Either way, it could impact your marriage negatively so you may have to face that and take into consideration whether you can cope with 5 children as a lone parent vs 3 children and some degree (great or small) of regret about a termination, should things go down that route.
All that said, I think you need to put aside all the “meant to be”, “twins are such a blessing” fluff- it’s fanciful nonsense. Twins are no more or less a blessing than a singleton pregnancy and a twin pregnancy is no more “fated to be” than any other. I understand being pregnant is an emotional thing, I’m not saying you should not have feelings about it- I’m saying the fairytale stuff and romantic notions aren’t going to lead to good decisions for your family. The issue you really need to focus on is whether you and your husband have the emotional, mental, physical and financial resources to adequately care for and provide for another 2 children, without significantly negatively impacting your existing children. That’s what really matters, that has to be your priority.
I would argue that your current housing set up is not going to work well for your children so you would need to consider the costs of a larger house, just as a starter. Then consider the additional costs of running a larger household, transport costs, clothing etc. can you do it? If you can’t, how do you think your current children will feel/cope, especially as they get older and need more space/independence? Will they be able to have room to study in peace, or able to have friends round etc? It is going to be very cramped with 7 people in a 3 bed-semi. You need to have understanding of the impact of expanding your family and if/how you deal with that. For example, overcrowded housing is associated with poorer educational outcomes- how are you going to mitigate against that?
I think you need to consider the impact on your existing children as a priority- and not in the “oh the eldest will simply LOVE to help out with the babies, she’ll be such a help” sense- again, this is a romanticised image and isn’t what you need to focus on. Your daughter might well enjoy baby siblings, but you don’t know that will be the case and you don’t know how long that might last even if she does initially (I.e. whilst it’s a novelty). You should not be looking to rely on your older children to help look after your younger children as a coping strategy. I’m not saying you are basing your whole decision on it, but whether she might like helping with babies is not relevant to your decision making process.
You need to be sure that you can be there to provide the support all your children will need- emotionally, spending time with them etc. will they have to sacrifice quality of life for their siblings (e.g. will you be able to support them in any hobbies or sports they do, or will they have to be curtailed or stopped)? If you honestly believe you can, then fine- if you can’t, then I think the existing children need to come before anything else.
If you get it wrong, you might well end up with resentment toward you and/or their siblings in the long run. Some parents manage large families really well. Some parents don’t. I have seen the results of both and with the latter there isn’t the lovely, jolly family all having great fun together and close siblings caring for each other (which is the picture I think OP is painting for herself)- it’s fractious relationships, resentment and quite a lot of emotional and psychological baggage for all involved. It can “all work out for the best”, and some people do muddle through and have a good outcome against the odds, but it doesn’t always and many parents don’t “just find a way”, so you need to have a realistic belief that you can do the right thing by everyone in the family before going ahead, imho.