To have told DD she has to come to family day out regardless of how she feels?

I agree. It's wrong the whole family misses out because she doesn't want to come. If she was left at home would this be really bad. I'd be tempted to do this unless I felt there was a huge risk.

I'm a bit baffled by the "why didn't you take her when she was first into Harry Potter" thing when the OP said her daughter didn't even read the books until she was 10 which is only 5 years ago and for a large part of those 5 years there would have been covid restrictions so they quite possibly wouldn't have been able to go even if they wanted too.

I know this isn't the main point point but as a far as busy attractions go ots massively autism friendly have quiet areas etc you can access on the way round well trained staff etc. Tool my autistic hp loving twins who had an absolute ball but one did have a wobble on the way round and staff were brilliant. Just incase that's a worry

Or now that both kids have enjoyed HP together, the parents decided it would be nice to take them so they can enjoy it together.
OP said she hadn't watched the films or read the books, maybe now both the kids can go and enjoy it and they both understand the sets etc it seems like a more reasonable trip than if only 1 of 4 people knew what's going on.
It's not a cheap day out, tickets, food, gift shops, snacks, transport and thats before potential accommodation.

Definitely not the case with my teens and their friends.

You sound like a lovely mum and a good loving family and I wouldn't take on any of the criticism for not watching HP films or favouring DS or any those reaches. Parents don't have to immerse themselves in their kids' interests, it's totally fine and quite healthy for them to have their own things. You've done a thoughtful thing and it's not gone down quite as expected for unknown reasons that your DD may even not be certain of herself. Being a 15yo girl is confusing enough without all the extra factors your DD deals with. That doesn't mean she shouldn't go or that she won't have a valuable experience one way or another. It is of course meant to be a fun family day out and hopefully it will be, but from babies through toddlers to teenagers, we all know that the dream experience we buy often has untold stresses on the day, and those make up memories and bonding experiences too. I think your instincts and approach is very sound and you'll go as a family and get something out of it. As you say, let her know it's booked and is happening and if she's got low expectations (like you and your DH) then so much the better for it likely exceeding them.

I think it's a shame you refused to take her when she was really into it, but now her DB wants to go, you can muster the energy. That would have upset me as a teenager. In fact, it would upset me now.
I would cut my losses and drop the family day, one of you go with DS.

teenagers can completely absolve themselves of any responsibility to others, such as opting out of family holidays at the age of 12. That's just utterly ridiculous.

So utterly ridiculous that my parents were happy with it. Responsibility to others isn’t being forced to do things you don’t want to, especially when it’s something that’s supposed to be fun - and that your mum refused to do on the same grounds.

Yeah, I've commented up thread in relation to op's specific situation. My comment here was in response to the person who essentially said that teenagers can completely absolve themselves of any responsibility to others, such as opting out of family holidays at the age of 12. That's just utterly ridiculous.

There are more complexities with the op's situation, I agree. If it were me, I'd probably be making plans for my dh to take ds on their own on this occasion.

Here's the thing... when I was a kid, if my parents booked and paid for something that I might not have loved nor my siblings, we had to go anyway because they had decided. I certainly didn't have the controlling vote nor did my siblings, if we weren't happy, we had to suck it up.
Not once has it made me think that I'm not in control of my life choices as an adult because they didn't pander to me. I'm a (reasonably) well adjusted adult.
Kids don't always know best, I think some adults need reminding if that.

So you didn’t go for years because you didn’t want to go, but now she doesn’t want to go but you expect her to suck it up and go?

Can you see that’s a little inconsistent?

Do not let your Daughter ruin this treat for your Son. One parent needs to stay at home with her.

No.
You don't force an autistic person.
You cannot be the "Silent Generation", so what the hell?
😖🙄

Being forced to go somewhere against your will isn’t doing something nice. It’s hardly selfish or entitled to say “It’s not my thing, please give my ticket to someone who’ll enjoy it and go without me”. Quite the reverse in fact. We’re constantly telling women not to “be kind” and put other people’s wishes before their own, those lessons need to start early.

If she's a HP fan, I would tell her she has to come.
I went this year with my daughter, we both love HP but even for someone who's not interested in HP, it still has lots of interesting information on the creation of the films.
How things were filmed to seem a certain way, costume and creature creations etc. Lots of interactive things to do and if that's still not winning, food court half way wasn't too bad 😉.
I would say if she uses ear defenders or has a comfort in busy or loud places then take them/it with you. The staff are really helpful too.

She has to go.
She can throw her weight around and make choices when she is 18. Right now she is a child and what the adults say goes.

Can dh take ds? You do something with dd.----

It is not well known. Lots of people say things about her but I have yet to be given an example of her bigotry. So can you find an example for me.

Oh and the Olympic boxer IS male. Biologically male. But the Olympic committee allows anyone with an 'F' in their passport to compete against women, or in this case to beat women.

So come on, find an example of when / where JKR has expresses extreme views on trans people?

The OP refused for years to take her daughter to HP. And this is supposed to be her daughter's birthday present. Is your birthday present often something you don't want?

What does JKR thinking Imane Khelif is male (I agree with her fwiw, as did Khelif's opponents & as does Khelif's coach...) have to do with trans people? Khelif's not trans, & was vocally offended at any such suggestion.

I'm not sure you're in a position to educate anyone. You seem confused.

Anyway, OP, I think I'd just send ds with one parent & sell the other tickets (unless there's anyone else you can think of who'd like to join). & take on board that you've now reached the stage in your family journey when family days out need to be planned by consensus...

You think standing up for the rights, privacy and dignity of women and girls is bigoted? And to champion the erosion of those things is ‘positive’? Oh dear…

If you’re teaching your teenagers that, then that is shameful and worrying. And what a pity they’re missing out in the JKR magic because of some warped worldview like that… 😔

Teenagers also need to understand that sometimes we do nice things for other people, because they do nice things for us. Selfish, entitled teenagers have a habit of turning into selfish, entitled adults.

You know no one believes that lie? Imane Khelif tested repeatedly as having a y chromosome. Imane Khelif is biologically male and everyone who hasn't got an agenda knows it. You are defending male repeatedly punching women in the face.

This btw, is why the youth are laughing at people like you now.