Yes. Have you? How many people with ASD do you know who simplycan'tbe left in their own home with anyone other than their parents, but would be perfectly fine at a typical wedding and reception? That isn't common, to put it mildly. Either she could be left with someone else if the parents wanted to go that route and grandma is just inventing reasons not to attend in an attempt to force the bride's hand, or the child is effectively guaranteed to have a meltdown at that wedding. I've seen more than enough full-on meltdowns to understand why no one wants that at their wedding. It's not like a toddler crying where they've usually forgotten about it and are perfectly happy five minutes later.
The problem isn't necessarily that she might "struggle" in that setting. It's even that by using "struggling" as a very transparent euphemism for "completely ruining everyone else's experience," you're attempting to shift the burden from herself and her parents to ensure appropriate behavior, onto everyone else to tolerate whatever happens because she's just "struggling." It's that her parents clearly don't view her "struggling" as anything they need to inconvenience themselves by attempting to address or avoid.
@PurpleOkapi
I guess you've heard the expression 'If you've met one person with autism, you've met one person with autism'. I do know someone like that, or at least a child - who can't be left with babysitters owing to anxiety - who'd like to attend a wedding, and who could, at least partially. No scenes at the church or reception, though the parents would be working hard in the background. I find your assumptions about autism worryingly prejudiced, and surprising for someone accustomed to the condition. There is nothing in the OP to indicate that the 11-year-old would disrupt the wedding with screaming meltdowns. And no, I am not using 'struggling' as a 'transparent euphemism for ruining everyone else's experience'. I'm using it for quiet distress of which only her parents might be aware, and which could be managed with some (or lots) of pre-planning on their part. I suspect if this child had the epic public meltdowns you're suggesting that she may have been diagnosed ealier. Some autistic people are very sociable and enjoy meeting people. That doesn't mean they find the experience easy or straightforward. It doesn't mean they want to be left out either.
This day isn't really about what the child wants though. As I said upthread, this is absolutely the bride and groom's decision and they should have the wedding they want. Nevertheless, I find it disturbing that you have changed the narrative from a couple simply wanting a child-free wedding to a couple who are actively discriminating against a little cousin who has autism.