I agree it's best not to be overtly hideous in response, yet I think the woman needs to understand how hurtful and inappropriate she is being/has been.
You need to decline (obviously) as it would be very bad for your self esteem to do that, in the light of not being welcome at the event itself.
You also need to make it clear why you're declining so don't use the response I posted yesterday (no, I'm saving the date) as it doesn't express the reasons.
She's clearly rather obtuse so it's going to take a little coaxing to get the right balance and get your message across.
Perhaps just, I feel hurt that you think I'm not close enough to share your occasion in any other regard than as a helper.
As such I don't feel I want to attend in any capacity. You may think this is sour grapes but honestly, would you not feel hurt at being asked to make it all lovely for the others while not being welcome yourself?
I hope it goes well but please don't expect me there.