I think it's a matter of what is better for OP,
It's a matter of what's best for the family of which OP is one of four, two of whom are existing children. She's really focussing this decision all on herself, and that's so very selfish.
her family being annoyed for a while when the baby comes
Annoyed for a while is one hell of an understatement. Directly going against the wishes of the other parent? Adding a third child to two that they can't support as it is? Abusing the good nature of her parents because she knows even though they think it's a bad idea and already bankroll her family, she knows when push comes to shove, they won't have the heart not to bankroll even more.
Her parents clearly love her so I think they'll support her with a new DC when it comes.
Her parents clearly love her, and she knows this, and banks on letting them pay for yet another child because she wants it.
Similarly, if he's not a complete twat, her husband will bond with their new DC.
Why? He doesn't want the child. He's been honest right from the start. And for all the right reasons. Just because it's born, especially under these circumstances, he won't automatically want it. That's a real, and awful possibility. Which again, OP is happy to risk.
Financially, people cope with a new DC, don't they? Cos they have to.
Financially, OP and her DH significantly can't cope with the two DC they've already got. And no, you don't have too. You can have a termination. And not disadvantage the existing DC even further.