Yabvu.
Your TWIN has said no. It's a milestone birthday on top of that. I think it's very unfair to make one day in a year all about you and disregarding any plans your twin sister may wish to make go celebrate her birthday.
Twin relationships are incredibly complicated because of the always having to share aspect.
On this occasion your twin sister does not want to share.
It does feel a little lazy on the part of your fiancé too. He's effectively saying he wants to celebrate your birthday and marriage together because, well, then he won't have to make arrangements for the other. All your friends and family will be there. It's one date instead of two for him to remember going forward.
And your sister is utterly sidelined for every milestone going forward.
It would be interesting to know the dynamic between the two of you growing up.
I have b/g twins and have to remind people they are twins. But when you're same sex or even identical, it's obvious and the two of you are forever paired in people's minds.
What is it which is driving you, or your fiancé, to dominate one day in the calendar so much?