It doesn't matter that they used an excuse. Well, no, scratch that, it DOES matter, because if that's an excuse, then what other truth is it covering?
Like, maybe they don't trust her driving? Maybe they think visiting a strange, extremely elderly relative will be upsetting for their kids?
The fact that it IS an excuse is all the more reason for Flora to go along with it.
As for your question--is there anything more that you can do: I don't think so. I think you've said what you can say. I wouldn't start putting your own judgment on anything, but simply stick with reminding Flora about the consequences she might suffer. (Which is what you've done so well, so far.)
Just be her "spare brain" and remind her of the consequences she might suffer if she goes ahead and does this.
I suppose you could encourage her to say to her son and DIL, "That sounds a lot like an excuse. I believe I'd be fine taking them, and though I might not enjoy my own visit w/ Dad, I can go again on another day--this visit would not BE about me spending time w/ Dad, it would be about the kids meeting him. I'm a grownup and can decide for myself how--and whether--I will "enjoy" my visit. Is there some other reason behind your refusal? If so, it would help me to know what it is."
But I don't get the impression that Flora could do that with any credibility.
And JenJay has another suggestion you could give her--in the spirit of being Flora's "spare brain" and "thinking of the things she hasn't thought of yet."
If it comes up again I'd say something like "I know it's important to you that your Dad get to see the babies, but I'm really worried what will happen if you go against your son and dil's wishes. I think you should talk to them again and ask if there is a time you can all get together and go visit your Dad."
(It may be simply that the parents don't want someone driving several hours away, with their children. I might not.)