OOOOH! 
Just when I thought she was never going to reply.
I think this is where you might use some or all of McBalls earlier suggestion. Which, to recap, was:
"Look, I could bullet-point all the ways your request is completely inappropriate but A) you have more important things to be thinking about right now and B) although I'm thoroughly taken aback by this I have no desire to further embarrass you.
I'm quite sure, once the excitement of the wedding has died down, you'll see that your request was very ill-advised.
No, I won't be available to help decorate but hope your day is wonderful."
I think you might be best to go with the last part, with a small addition.
"I'm quite sure, once the excitement of the wedding has died down, you'll see that your request was very ill-advised.
No, I won't be available to help decorate, and I will not be changing my mind, but I hope your day is wonderful."