helliebean, Tidy has already spelled it out in an email to the sister, last Thursday
"Dear Sister,
I'm not entirely sure myself why F is upset if I'm honest. All I can do is really explain my position and hopefully it'll make sense to you.
Your sister invited me and [DP's name] to her wedding by way of a Save The Day card, and I made plans accordingly. I've since learned that I'm one of 20 who received the original save the date cards and whom are not invited after all. I've accepted this as I appreciate that every bride has budgetary constraints and family pressures that change over time.
What I didn't expect was then to be asked to decorate her venue, as F had never discussed this with me before less than a fortnight ago and I'm afraid it's just not possible for me to help for a number of reasons. I'm not sure if you're under the impression that I had arranged to help F and then pulled out, but this isn't what happened. I truly had no idea she was going to approach me to do this until I received her email.
Your sister is normally so thoughtful, so I'm surprised she hasn't realised what an imposition it is. It's just too much I'm afraid and I simply can't help on this occasion.
Hope all goes well and look forward to seeing the pictures in due course.
Kind regards
Tidy"