Dad refusing to accommodate SEND daughter's needs at wedding

Why dont you have a formal contact order in place?

You don't let either child attend the wedding. Because you are a mother. A mother's job is to make decisions in the best interests of their children, with all the facts available. Children can't make these decisions because they have no idea what complex issues are being played out.

It isn't fair or right that one of your children doesn't get to go. So neither of them go. That's not on you, it's on him.

Glasschild syndrome may be a 'thing' (I have 3 with SN) but it's more important for the child who is NT to know that we don't allow people to discriminate against their siblings on our watch. That doesn't mean that they always get to do everything they want to do. There may be legitimate situations where they aren't invited, or it isn't appropriate. But this isn't one of them. This is a situation where a child is being bullied and to participate in that and collude in it would send a terrible message to them both.

there is domestic abuse, coercive control and a history of manipulative and verbal abuse towards both children from Dad

There is no court order in place.

Hasn't seen SEND for over a year. NT sporadically

She's not attended for contact in over a year, due to inflexible rules. Added to that messages since calling her ungrateful and rude

there has been a severe breakdown in the relationship for both children.

Having read all that, it's a no brainer for me. Of course you can't allow one child to attend and not the other, you're mad to entertain the idea. His inflexibility is appalling. Given that he's only had sporadic contact with your NT child, it's completely inappropriate for them to have a prolonged visit over the time of the wedding, anyway, even if ND child was going.

Tell the children that neither are going and it's not up for debate. Plan something really exciting - preferably something that gives you an excuse to buy them both a special outfit to wear (if that's what DC2 was excited about). DC2 will be disappointed but you have to tell them that it's wrong to exclude people - and excluding disabled people is discriminatory - which is why she can't go. You must be very clear that it is NOT DC1's fault that DC2 can't go, it is their father's fault for excluding them and for refusing all the possible solutions you've suggested.

Also, I don't see why NT child should have to see him at all. There is no court order so you won't be in breach of anything. It sounds as if contact with him is not in her best interests.

Do not allow him to continue to abuse your children in this way.

Absolutely right.

There are lots of things that DC want to do - but are not in their best interests. You need to take control of the decision here - neither should go. Yes, both will be upset and one might be angry, but I’d book something else nice and take both. I’d say one more time that it is not right that they are not both invited. If his response is the same, then I’d tell him that neither will be going. If his parents get involved, I would treat them to a discussion of their son’s disgusting approach to parenthood.

He sounds the type who will react badly if you say to him neither are coming. You're going to have to find a way to make him feel ok with the fact both are not going.
Part of me thinks you should be honest with the 9 year old NT. Tell them a version of the truth that says look for these reasons this is difficult and instead we will be doing x,y,z on that day.
What would you guys both like to do that weekend?
I'm sorry ( NT kid) but this is beyond my control.
It isn't right that you can't both go here so we're doing something else.

It is the truth and you aren't criticising him. NT may resent ND. Either way it's a lose lose on some level.

Handling him will be the problem. He probably will care more about you telling him no than the fact his kids won't be there so that needs gentle handling for an easier life.

You were trying to facilitate your children into a situation to likely get verbally abused by their father?

I would take them somewhere else then on the day of the wedding. What would would happen if NT child gets the wobbles after a few hours/days and wants to come home early, if they go to the wedding?

what he is doing is a continuation of his abuse. He is knowingly making it impossible for your SEND child to go and then pitting the siblings against each other.

Neither goes. It will be difficult now but they’ll understand as they get older.

Isn’t it likely that if SEND DC goes (under dad’s preferences) they will end up unhappy /not coping and you will be summoned to come pick them up anyway?

It seems dad just wants them there as he knows it will look bad for him otherwise, but he’s not thinking about their needs.

This was last year, first one post separation.
the kids never heard about it. I did try and encourage them to go because I didn’t mind having Xmas lunch another time but they were adamant they didn’t want to.
I tried to negotiate with him and suggested brunch, dinner, etc.
I invited him to my parents house with us for Xmas lunch but he declined.
in the end he realised that despite threats and lashing out verbally he couldn’t make them come, so we went up together Christmas Eve and I also visited with the kids Christmas Day afternoon. They did get their presents of course. That’s how he is, but in the moment he’s very convincing and it’s very upsetting!

Yes both children should be allowed to go, he's being a bad father for not facilitating it. Are you willing to drop off then collect on the day?

I have a dsd with very high level needs (also autistic but the least of her problems) and we still arranged for her to come including paying her care team overtime!