Just wondering if maybe they are in an area that has time share. Then, you wouldn't be houseguests. Might even think of renting a motor home to drive down in, or rent when you get there.
Park in the drive way, plug into the 220...leave a little something for the electric bill.
Great way to get some alone time with the grand babies.

You sound like me, with the good-byes, I'm not much for them myself. Last time my brother came to visit, he was up and out at 4 in the morning, no good-bye. He dislikes the goodbyes, just as much. Either that or he just didn't want to face my crying. Come to think of it...time for another visit. Years pass to quickly.

Not sure if this is the case or not, but, it
sounds like your DIL isn't to keen on traveling outside her comfort zone. If, that is so...it makes things more difficult. But, not impossible.
Is she open to new ideas?
Like maybe...each of you traveling half way and staying for a week in new territory? That way no schedules, to meet. Just thoughts to ponder.

You have a mother's heart...you think of everyone else when you process thoughts and feelings...much love there.