@Neapwind I think in the longer term your empathy will be a huge asset to your younger DGD and goodness knows, she will need it. It is a difficult time immediately after diagnosis, for all involved. You are clearly not being a bitch to anyone and if you think your bride-to-be DGD will be upset and it will help her understand, I would be inclined to arrange to meet with her, just the two of you to talk and to listen. Not only about the wedding day situation, but also that you are finding it hard learning this new information about her younger cousin and that you’re feeling conflicted about her (the older DGD’s) special day. She may really appreciate the chance fir someone one to one time with you to talk about how the planning is going outside of this issue. She may not even have the slightest inkling that you are worrying about all this. Planning a wedding is stressful! But it is a DAY and I do think the wedding hysteria can be all consuming (most definitely for many on this thread).
I think it’s essential that you have it straight in your mind that you’re not attempting to change their minds about inviting DGD11, it will not go well if this is your aim or even hope. She doesn’t have to agree with your choice but just as she is free to choose, as are you. I dint imagine she is deliberately setting out to cause hurt and upset with her choices and you need to try and express that equally, neither are you.
Those who keep spouting on about things not revolving around the 11yo DGD, please believe me when I say that any autistic child or young person (or adult come to that!), discovers a thousand times a day, every day, that the world does not revolve around them. I thought I knew what autism was about before I had my autistic DDs. I did not. If you don’t live with this closely in your life, you really don’t know either and compassion is the ONLY path. A lot of other things such as infinite strength, boundaries and the rest too are needed, but compassion is above all else.
Send your bride DGD a beautiful gift on her wedding day, tell her you understand her wishes (even if they wouldn’t be your choice) and mean what you say, but maybe you can help turn the world around your 11yo DGD for a few hours on that day too.
Proceed with kindness
good luck.