V.W.
I rarely even left the house before my kids were 6 weeks old. There are just way to many germs and other factors that are hard to avoid in public, let alone and confined airplane. Not to mention how exhausted she will still be in December.
I am sure this is a very difficult decision for your family, however, as your daughter is about to find out, kids will change many social plans over years. I am sure with her waiting for so long for this baby, she will find the reward of motherhood is worth missing the party.
May your family be blessed for your understanding and sacrifice. Congratulations on a new grand baby!!