I think the idea behind it is so Tidy doesn't look like she still wants an invite Longtalljosie.
Her perfectly reasonable decision not to help could be twisted to look like sour grapes or angling for an invite if she keeps mentioning it.
I think MrsKoala is right about the the childcare comments actually, so I'm changing my stolen from Horry suggestion again to:
Dear Sister Of The Bride
I really don't understand myself why Gluezilla is upset, so perhaps you could explain it to me.
She invited me to her wedding last year by way of a Save The Day card, and I made plans accordingly. I've since learned that I'm not invited after all, which I've accepted 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 Gluezilla had never discussed this with me until X date. I'm afraid it's just not possible for me to help for a number of reasons.
I can't believe Gluezilla doesn't realise what an imposition this is given how thoughtful she normally is. 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