i think there’s ways to accommodate the situation for DC and we can limit the time DC is present and can go back to the hotel room frequently.
The issue is now that I don’t think sister actually wants me anywhere near her wedding so all of this planning is for nothing!
DH and I had come up with a reasonable plan but now he’s seen this behaviour from her come up again, I don’t think any of us will be going.
What a shame, OP, all this drama when you've got your newborn to enjoy precious first days with, and your older child and DH too.
How about emailing your sister and copying in your mum, to say 'we're so happy for you and would love to attend your wedding as a family but as you know, older DC has special needs and we don't want anything to spoil your special day. So we've come up with these arrangements where DC will attend for a short while, and maybe, depending on how they cope, can be in some of the photos, but DH will be on hand to take DC back to the hotel if it all gets too much for DC. I'll attend with baby, and stay for as long as baby's needs allow.'
'How does that sound, DSis? Do you want us to attend on that basis, or would it be easier for you if we don't come, because obviously we must put the needs of our DC first, no getting around that, I'm afraid. When you have your own children, you'll see how, like it or not, their needs must come first. Thanks again for inviting us, we're so pleased for you and fiance.'
Should she stamp her feet and say no you can't come or no DC must be there all day, then you'll have evidence of her unreasonableness to trot out on future occasions.Then, whatever the outcome, after the wedding go low contact with her - she'll have her new life to be getting on with, and having made a stand in favour of putting your family's needs first, you can continue to assert yourself.
I know many are saying just ignore her, but it is a wedding and it is a special occasion, so it's worth offering a gentle compromise. Who knows, she might accept what you say and DC attends for a while and enjoys it. If not, it's her loss.