Am I being unreasonable?

Ladies, I need some outside opinions. 
My husband's aunt has graciously offered to throw a baby shower for me - something I didn't expect but am very grateful for. 
Aunt is very set on having my husband's grandmother attend the shower. Again, not a problem until you realize that his grandmother will be out of town for weeks 35-40 of my pregnancy. This was a planned trip and was a surprise to no one.
The problem comes with timing. We are now 20 days away from when grandma leaves town and no invites have been sent. A date has supposedly been picked, but I'm not sure when exactly it is. I'm guessing some family knows, but none of my friends do and I feel like this is no longer a party for me & my baby. Part of me doesn't even want to go. 
Am I being unreasonable? Any suggestions on what I can do to just be at peace with this?

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