Ah well this is just kind of script. You didn't want to help,(or couldn't, rather) so she put the please and smilies on you, then you still said no (she didn't care about all the 'why' stuff) and then she told someone else to 'have a word' with you to make you repent and agree to help.
You don't have to respond. You have a couple of options though if you do.
For examples,
- 'Hi there, thanks for passing that on.
I'm sorry, I didn't get the impression your sister was upset at all. I will discuss it further with her if she wishes but I am still unable to help, etiquette issues notwithstanding.
Regards'
- Hi there. I think there has been a misunderstanding, I'd rather just sort it out with bridezilla to keep things clear. But thanks for letting me know.
- Hello,
I understand she was fully expecting me to drop everything to help, however I felt uncomfortable at the idea of being asked to leave before the actual event as I'd always hoped I was a close enough friend to be invited to her wedding.
Hope this clarifies a bit. Please pass on my best wishes to Bridezilla,
Regards'
and so on and so on