"No siblings would mean either DH or I missing the show."
No it wouldn't, because alternative date was offered, for siblings to attend.
Usually this is done to be able to space the chairs out differently, to allow for prams.
I agree, the Nan could if took the youngest to the day time performance. Surely if she is recovering from spinal surgery, having a toddler to manage two days afterwards wouldn't be ideal anyway.
To turn up with her Mum and cause a fuss, is bizarre.
Saying her DD wouldn't perform without her younger brother there is first class PFB behaviour, the rules are the rules and apply to her children.