Your husband is right to tell his sister she would have to choose between the ex being invited, or the two of you. If they can’t respect your wishes and needs, then you have to honor your own and take a stand for what you believe. I wouldn’t belabor the point. If he has already let his sister know, I’d put a time limit on the decision process. In other words, something that says “if we don’t hear from you by…we will assume you’ve made your choice and will not make arrangements to be there.”
Finally, if you have it in you and it is authentic, let his younger sister know that it is important for her to have her special day the way she wants it, and that you wish her well even though the situation is such that you won’t be there. It sounds like both these things are true, and if they are, you can say them as a way of maintaining a connection.