Ok, you asked DS what the problem was and he told you. You may not agree with some of DIL's feelings, but those are her feelings. If she needs more space than you expect, she's going to keep needing it whether you agree or not. If she feels you and DH are "under her feet"too much when you visit, she's going to continue to be bothered by that even if you're thinking, "oh please." So I agree with the idea of booking yourself somewhere when you visit them. I would also add, limit the number of hours you visit. Maybe find some things to enjoy on your own after you leave them like a show or dinner out. Or, at least, plan some outings with DS and your GDs and accept it if DIL doesn't come. You're not really there to see her anyway, are you?
It does seem sort of rude for her to go off and read a book when she visits. However, if she's a little introverted and needs that space, probably best to just accept it. Same with her not Skyping. Again, please remember, the important thing is that you have time with DS and your grands.
In other words, change your own behavior where you can (booking a room, saying no more about her absences), but stop asking about or hoping to change hers. That's where the "do nothing" part comes in, I think, and I'm glad you've already decided that it is a good idea.