*She isn't a parent.
She's referred to a diagnosis of 'mild autism' (whatever that is) and then decided she's 'very sad' to the point that it requires her to hijack a family member's wedding over it.
Pull the other one*
I agree with @tigerinyourtank. A diagnosis of autism is an adjustment and can be a lot to absorb. However, as Tiger and PP have observed, the OP seems to be making this wedding about her own needs and wailing about the diagnosis of autism.
From the OP's description of "mild" autism - which isn't a thing by the way - and no behavioural problems there's no associated learning difficulties. Yes, autism can have devastating consequences when there is associated cognitive impairment but that isn't the case here.
I'm autistic and my DC are both autistic. We don't need pity and I don't think hand-wringing over a diagnosis is fair on anyone. I'd suggest OP learns a bit more about autism because her description and ideas sound very out of date. Perhaps if she was more informed she wouldn't be so utterly devastated and worried. Many autistic people lead fulfilling, happy lives - some of us even manage to pass for one of you normal folk, there's more of us than you might think... 😘
Is it a shame that the 11yr old can't attend? Of course. And if she was expecting to go she'll be disappointed. I feel for her, of course I do. But at the same time, the granddaughter is entirely free to have the wedding she wants. Maybe others have asked to bring their DC and she's told them it's a child-free wedding - once you allow one child to attend, you start causing problems with other guests.
Autistic children normally do quite well with facts which are clearcut and easy to understand. There are no other children at this wedding, so that's the only reason she can't go. Very black and white. Also, as a softener, could you suggest to granddaughter that you do a separate wedding meal that the little girl could go to?
What's needed here are solutions and unemotional explanations rather than reinforcing the idea that this is all so unfair.