This is the thread that keeps on giving! 
I'd keep it short and sweet.
Reply "Did you know I am not invited to this wedding, despite being sent a save-the-date card?"
If she replies with Erm no you can come back with "Well hopefully you'll see why I think it's rude for X to ask me to decorate the hall for people she HAS invited, especially as save-the-date cards are only supposed to be for people you intend to invite."
If she replies with Yes I did come back with "so you think it's OK for X to ask me to decorate the hall for people she HAS invited, especially as save-the-date cards are only supposed to be for people you intend to invite?"
If she starts with But she's sooooo upset! I'd suggest - "so am I actually and it's getting worse that she doesn't see how rude she is being, has not apologised, keeps asking, and now has got you involved, which is frankly astonishing."
If she starts with But they can't invite everyone then say "That's no problem. Her guest list - her choice. But by the same token: My time and free labour - my choice."
AND FINALLY
You could remind her:
It is never a good idea to clearly demonstrate to someone how little they mean to you, just before asking them for a massive favour.