To prioritise myself over my daughter here?

But you’ve kind of answered your own question here?

They don’t initiate contact at any other time and neither do you.

So go to the dinner and forget about them the rest of the year?

why is that not an option?

You can take your daughter to the Christmas dinner and pick her up, without entering the house or speaking to anyone. I think that while she is still under age, you do need to facilitate this for her if she wishes, but you do not need to put yourself in harm’s way to do so. DD has to choose whether she wishes to go alone or not at all, but she does not get to dictate that you go.

I wasn’t suggesting she should be

This actually is now a question of whether or not you are prepared to draw a boundary with your daughter, as well as with your mother.
I have 2 DDs with AuDHD and I do understand that this is not the same as drawing a boundary with a neurotypical teen - but that doesn’t mean that you should not.
She has decided that she wants to go to the annual family dinner, which is fine.
She has decided that she wants you to go too, which is not fine.
She is becoming an adult and needs to start to see her role as distinct from yours.
She won’t like it if you say “I will not be going anymore. It is the right decision for me. You can make your own choice but you do not get to choose for me” but actually it’s a powerful lesson and important example to set. She needs to understand that you and she will not always agree. She has agency over her own actions but not yours.
Rigid autistic thinking and fear of change may well have her maintaining that she “cannot” go without you but that doesn’t make it true. It will be uncomfortable. You will get push back and maybe meltdowns but it’s also important for us to learn to be comfortable with our autistic adult children’s discomfort. We cannot fix everything and they need to internalise that.

Unless actual risk of damage to DD, then it seems hugely disproportionate. I’m sure you can civilly get through the occasional family do, or come up with some sort of reasonable arrangement.

Then she doesn't go. Too bad.

DD has said she won’t go without OP

Well if DD refuses to go to the once a year dinner (weird in itself as no contact at all the rest of the year.....) then im afraid its too bad.

Keep away from your horrible mother. Don't go to this ridiculous "get together".

Time DD made efforts to make new friends - you won't live forever so she needs to start standing on her own 2 feet.

Ask to be clear they are saying either you BOTH go or your dad isn’t invited?

I’m not sure I understand the problem then. DD isn’t going if you don’t go. You aren’t going, so neither is she.

DD has to understand she doesn’t get to control where you go or who you see. She gets to decide that for herself, but not for other adults, including her mother.

From your posts it sounds like the rest of the family are keeping their distance; at the dinner, does your DD chat or interact with her relatives? Could she keep contact via social media?

Just give it a miss this year and see how you get on.

I’m sorry I must have missed that your DD won’t go without you. Apologies.

Op it’s possible she sees this dinner as a way she belongs to a big family- when in actual fact it’s the breadcrumbs they throw her one day of the year.
on every other day, she isn’t included and that is not acceptable.

I would have an honest conversation with her. She’s 16. Tell her you support her, she can go BUT she goes alone. No need to torment yourself like this.

She’s at an age where she should be able to understand. I wouldn’t even want her to go, but again it’s her choice. So she goes but she goes alone.

Why does she want to continue going to the Christmas dinner? Is it more to continue a childhood tradition, or to see the people there?

Is it painful to her that aside from this dinner, she's ignored by family for the rest of the year?

Is it in her best interests to be removed from relationships with people who don't have her best interests at heart?

If I were thinking of going no contact with family, there'd be a very good reason for that. And I doubt I'd think of that person as an emotionally safe person for my child to spend any time with. I would want to protect them from that person more than I would want to protect myself from them. I absolutely couldn't imagine going no contact with someone and feeling OK with my child continuing to see them

I have a mother like you, and I think it's a really bad idea to cut off your whole family - for your daughter's sake. I always wanted to have family around me, but my mother didn't and would always find ridiculous reasons to keep everyone at arm's length (until she needed something from them) - so I hardly grew up with any family around, and I resent her for it. I see everyone having big family gatherings and popping in at aunties' for tea...I don't have that, really. At least I've got my siblings but your daughter doesn't even have that. Don't let her grow up so alone just because you, for whatever reason, can't be arsed.

As far as I can see, the family has not invited the op. They have invited the DD.

and the DD isn’t asking the op to go with her

so what’s the issue? Your DD goes to the lunch not on Christmas Day!

OP you cannot go to the dinner. Your daughter is old enough to go alone if she wishes but you definitely should not go.

A Christmas dinner sounds like a key tradition for your DD and as you know, autistic people hold on to routines hard.

I presume you've explained the implications of keeping the relationship going in order to attend?

Is your family saying that unless you attend, your daughter can’t attend?
No

So what’s the big deal. Your DD goes to this family meal not on Christmas Day. You do your own thing for a few hours, your DD comes back. Drama over. Done

So you are moving very very imminently this is for college in September

Why do they exclude you from everything else bar this one dinner? It's one dinner once a year, I'm pretty sure you can handle having hardly anything to do with any of them all year apart from this for the sake of your disabled daughter. I know it would feel real nice to cut your mother off, but you might just have to wait a few more years.