Wow, this sure does sound difficult! I am not a fan of long drives, and this is REALLY long. And having the elderly parents-in-law in the car sounds much worse. I’m glad you found affordable accommodations at least, so it is not such a huge financial burden. But I agree, much of this sounds extremely unpleasant to me.
That being said, I think no one but your daughter will know how she’ll be feeling at that time. Chemo reactions definitely vary not only by type of chemo but by the person. I myself went through a huge 6 month course of some of the worst types of chemo (followed later by another 6 months of a much milder chemo), and while it affected me, I was on CLOUD NINE going to my aunt’s big fancy 80th birthday party in NYC 5 days after my last infusion for which I had to travel and get dressed up and we couldn’t get a cab so I had to walk like 10 blocks in high heels. I’m not saying that I was in such unbelievable shape and that chemo is no big deal. It IS a big deal, and physically I felt somewhat lousy. But the sheer joy I felt at being done (or so I thought, ha ha ha) propelled bursts of energy and I just LOVED being surrounded by my large extended family. Absolutely worth it to me, one of my favorite nights and gatherings of my life–I truly was on cloud nine at that point, and getting to share that euphoria with people you love is awesome. I had also insisted on hosting Thanksgiving for our family of 30 that year, about a month and a half later, which was 5 days after my surgery. It was something really joyful and positive that I could be excited about, and I absolutely loved hosting it, even though of course others offered to host. It took extra effort as I had to set a lot of things up before my surgery and I had to be careful about how I moved, so everything took me longer than usual, but another highlight of that year for me!
There is a range of how people handle the fact that chemo does a number on your immune system, and there’s also a range of what doctors will suggest about what you do during this period. I know I was less cautious than many, and for me the risk/reward tradeoff had that make sense as I would have been in such a mental slump if I had squirreled myself away during that time. For other people, the tradeoff analysis would be different. Certainly if someone had obvious flu symptoms or whatever, I would not have hung out close to them, but otherwise I was comfortable being around people who appeared healthy (nowadays n95 masks are so readily available, that if there’s a concern, she can don one while at the gathering as a good way to mitigate risks). I would suggest everyone in the group take several covid tests leading up to the gathering to lower the odds of covid going around (and since this is a beloved daughter going through this, I’m sure it could be worth it to many of her family to avoid crowds/unnecessary gatherings for several days leading up to this family occasion).
So all of that is to say that I think how she’s feeling and whether she’ll want to be with family is really her decision and I wouldn’t worry about that part. It’s my belief from what OP wrote that this daughter REALLY wants this to happen and I have a hunch that she, too would feel on cloud nine to have this celebration, so don’t worry about whether she’ll be tired from chemo.
However, all that being said, I am surprised at how little concern she is exhibiting at requesting so many people inconvenience themselves in a such a massive way (14 hour drives for old people!!! Egad!! I sure hope the 14 hours includes lots of built-in breaks, and hopefully it’s really more like a 10 hour drive plus lots of meal stops??). To be honest, I would personally never in a million years push for that. But, it has clearly been a very scary and emotional year for her, so maybe she gets a pass this one time (but in my book, SHE’s the one traveling for Christmas for the next 10 years!).
Good luck, I think there’s been a lot of good advice above about taking care of yourself and managing your own anxiety. I hope that if it happens, it turns out to be a memorable and wonderful time, and worth the effort!!