I like Horry's suggestion but with one alteration, if you haven't already replied.
Take out the bit in bold - but still not be invited to share her special day - as it could still look like you are angling for an invitation in exchange for your help.
And possibly take out this bit - but maybe she will once she has children of her own - as well.
Dear Sister Of The Bride
I'm sorry to hear that Gluezilla is upset. I thought we had come to an understanding.
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 but still not be invited to share her special day. I would have had to rearrange time off work (which I had cancelled once I knew I wasn't invited) and expensive childcare in order to do so.
I can't believe Gluezilla doesn't realise what an imposition this is given how thoughtful she normally is, but maybe she will once she has children of her own. 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