The original plan for GMIL's actual birthday, according to MIL several months ago, was immediate family who were local to the city (so roughly have my direct IL's only, and not any of the extended aunts uncles, etc) would gather at GMIL house for a cake.
Instead, she didn't tell anyone and took a cake to GAIL house at her annual family gathering, and celebrated with extended family from one side there. then on GMIL actual bday, invited the rest (and majority) to GMIL house for a big pot luck dinner celebration, with a huge cake.
DH nor I have access to these extended relatives. We have never lived in their city or close to it (DH left home at 17 and never went back). We don't have their phone numbers. That was a huge reason we were going to all this trouble too - to try and get closer to these relatives who MIL always (for some unknown reason) 'gatekeeps' us from. This would be the first time in over a decade we'd even see them, and we really want to get in touch with them. (We will be asking AIL, when she's feeling better, to provide us with addresses and contact information so we can just speak with them directly in the future)
MIL did not cancel the venue. What she did though, was have these two seperate events with all these people who DH and I have no current access to - and one poster is dead on; this is how it always is in their family. Everything goes through MIL. I'd be making people VERY uncomfortable and MIL would create massive drama for everyone involved if I tried to piece together this get together. MIL actually told me she NEVER bothered to tell these family members about the event - the only one's who knew were the one's AIL contacted. Word got around, and MIL got some phone calls, and told people not to bother coming, and to just come on the actual bday instead.